Merge pull request #10 from systemli/Add-Prometheus-Metrics

🚸 Add Prometheus Metrics

README.md

@@ -14,6 +14,7 @@ The adapter is configured via environment variables:
 - `DOMAIN_LISTEN_ADDR`: The address to listen on for incoming requests. Default: `10002`.
 - `MAILBOX_LISTEN_ADDR`: The address to listen on for incoming requests. Default: `10003`.
 - `SENDERS_LISTEN_ADDR`: The address to listen on for incoming requests. Default: `10004`.
+- `METRICS_LISTEN_ADDR`: The address to listen on for metrics. Default: `10005`.
 
 In Postfix, you can configure the adapter as a transport like this:
 
@@ -23,3 +24,44 @@ virtual_mailbox_domains = tcp:localhost:10002
 virtual_mailbox_maps = tcp:localhost:10003
 smtpd_sender_login_maps = tcp:localhost:10004
 ```
+
+## Metrics
+
+The adapter exposes metrics in the Prometheus format. You can access them on the `/metrics` endpoint.
+
+```text
+# HELP userli_postfix_adapter_request_duration_seconds Duration of requests to userli
+# TYPE userli_postfix_adapter_request_duration_seconds histogram
+userli_postfix_adapter_request_duration_seconds_bucket{handler="alias",status="success",le="0.1"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="alias",status="success",le="0.15000000000000002"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="alias",status="success",le="0.22500000000000003"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="alias",status="success",le="0.3375"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="alias",status="success",le="0.5062500000000001"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="alias",status="success",le="+Inf"} 1
+userli_postfix_adapter_request_duration_seconds_sum{handler="alias",status="success"} 0.074540625
+userli_postfix_adapter_request_duration_seconds_count{handler="alias",status="success"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="domain",status="success",le="0.1"} 3
+userli_postfix_adapter_request_duration_seconds_bucket{handler="domain",status="success",le="0.15000000000000002"} 3
+userli_postfix_adapter_request_duration_seconds_bucket{handler="domain",status="success",le="0.22500000000000003"} 3
+userli_postfix_adapter_request_duration_seconds_bucket{handler="domain",status="success",le="0.3375"} 3
+userli_postfix_adapter_request_duration_seconds_bucket{handler="domain",status="success",le="0.5062500000000001"} 3
+userli_postfix_adapter_request_duration_seconds_bucket{handler="domain",status="success",le="+Inf"} 3
+userli_postfix_adapter_request_duration_seconds_sum{handler="domain",status="success"} 0.246158083
+userli_postfix_adapter_request_duration_seconds_count{handler="domain",status="success"} 3
+userli_postfix_adapter_request_duration_seconds_bucket{handler="mailbox",status="success",le="0.1"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="mailbox",status="success",le="0.15000000000000002"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="mailbox",status="success",le="0.22500000000000003"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="mailbox",status="success",le="0.3375"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="mailbox",status="success",le="0.5062500000000001"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="mailbox",status="success",le="+Inf"} 1
+userli_postfix_adapter_request_duration_seconds_sum{handler="mailbox",status="success"} 0.097836333
+userli_postfix_adapter_request_duration_seconds_count{handler="mailbox",status="success"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="senders",status="success",le="0.1"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="senders",status="success",le="0.15000000000000002"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="senders",status="success",le="0.22500000000000003"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="senders",status="success",le="0.3375"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="senders",status="success",le="0.5062500000000001"} 1
+userli_postfix_adapter_request_duration_seconds_bucket{handler="senders",status="success",le="+Inf"} 1
+userli_postfix_adapter_request_duration_seconds_sum{handler="senders",status="success"} 0.097870375
+userli_postfix_adapter_request_duration_seconds_count{handler="senders",status="success"} 1
+```

adapter.go

@@ -6,7 +6,9 @@ import (
 	"fmt"
 	"net"
 	"strings"
+	"time"
 
+	"github.com/prometheus/client_golang/prometheus"
 	log "github.com/sirupsen/logrus"
 )
 
@@ -27,26 +29,32 @@ func NewPostfixAdapter(client UserliService) *PostfixAdapter {
 func (p *PostfixAdapter) AliasHandler(conn net.Conn) {
 	defer conn.Close()
 
+	now := time.Now()
+
 	payload, err := p.payload(conn)
 	if err != nil {
 		log.WithError(err).Error("Error getting payload")
 		_, _ = conn.Write([]byte("400 Error getting payload\n"))
+		requestDurations.With(prometheus.Labels{"handler": "alias", "status": "error"}).Observe(time.Since(now).Seconds())
 		return
 	}
 	email := strings.TrimSuffix(payload, "\n")
 	aliases, err := p.client.GetAliases(string(email))
 	if err != nil {
 		log.WithError(err).WithField("email", email).Error("Error fetching aliases")
 		_, _ = conn.Write([]byte("400 Error fetching aliases\n"))
+		requestDurations.With(prometheus.Labels{"handler": "alias", "status": "error"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
 	if len(aliases) == 0 {
 		_, _ = conn.Write([]byte("500 NO%20RESULT\n"))
+		requestDurations.With(prometheus.Labels{"handler": "alias", "status": "success"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
 	_, _ = conn.Write([]byte(fmt.Sprintf("200 %s \n", strings.Join(aliases, ","))))
+	requestDurations.With(prometheus.Labels{"handler": "alias", "status": "success"}).Observe(time.Since(now).Seconds())
 }
 
 // DomainHandler handles the get command for domains.
@@ -55,10 +63,13 @@ func (p *PostfixAdapter) AliasHandler(conn net.Conn) {
 func (p *PostfixAdapter) DomainHandler(conn net.Conn) {
 	defer conn.Close()
 
+	now := time.Now()
+
 	payload, err := p.payload(conn)
 	if err != nil {
 		log.WithError(err).Error("Error getting payload")
 		_, _ = conn.Write([]byte("400 Error getting payload\n"))
+		requestDurations.With(prometheus.Labels{"handler": "domain", "status": "error"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
@@ -67,15 +78,18 @@ func (p *PostfixAdapter) DomainHandler(conn net.Conn) {
 	if err != nil {
 		log.WithError(err).WithField("domain", domain).Error("Error fetching domain")
 		_, _ = conn.Write([]byte("400 Error fetching domain\n"))
+		requestDurations.With(prometheus.Labels{"handler": "domain", "status": "error"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
 	if !exists {
 		_, _ = conn.Write([]byte("500 NO%20RESULT\n"))
+		requestDurations.With(prometheus.Labels{"handler": "domain", "status": "success"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
 	_, _ = conn.Write([]byte("200 1\n"))
+	requestDurations.With(prometheus.Labels{"handler": "domain", "status": "success"}).Observe(time.Since(now).Seconds())
 }
 
 // MailboxHandler handles the get command for mailboxes.
@@ -84,10 +98,13 @@ func (p *PostfixAdapter) DomainHandler(conn net.Conn) {
 func (p *PostfixAdapter) MailboxHandler(conn net.Conn) {
 	defer conn.Close()
 
+	now := time.Now()
+
 	payload, err := p.payload(conn)
 	if err != nil {
 		log.WithError(err).Error("Error getting payload")
 		_, _ = conn.Write([]byte("400 Error getting payload\n"))
+		requestDurations.With(prometheus.Labels{"handler": "mailbox", "status": "error"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
@@ -96,15 +113,18 @@ func (p *PostfixAdapter) MailboxHandler(conn net.Conn) {
 	if err != nil {
 		log.WithError(err).WithField("email", email).Error("Error fetching mailbox")
 		_, _ = conn.Write([]byte("400 Error fetching mailbox\n"))
+		requestDurations.With(prometheus.Labels{"handler": "mailbox", "status": "error"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
 	if !exists {
 		_, _ = conn.Write([]byte("500 NO%20RESULT\n"))
+		requestDurations.With(prometheus.Labels{"handler": "mailbox", "status": "success"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
 	_, _ = conn.Write([]byte("200 1\n"))
+	requestDurations.With(prometheus.Labels{"handler": "mailbox", "status": "success"}).Observe(time.Since(now).Seconds())
 }
 
 // SendersHandler handles the get command for senders.
@@ -113,10 +133,13 @@ func (p *PostfixAdapter) MailboxHandler(conn net.Conn) {
 func (p *PostfixAdapter) SendersHandler(conn net.Conn) {
 	defer conn.Close()
 
+	now := time.Now()
+
 	payload, err := p.payload(conn)
 	if err != nil {
 		log.WithError(err).Error("Error getting payload")
 		_, _ = conn.Write([]byte("400 Error getting payload\n"))
+		requestDurations.With(prometheus.Labels{"handler": "senders", "status": "error"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
@@ -125,15 +148,18 @@ func (p *PostfixAdapter) SendersHandler(conn net.Conn) {
 	if err != nil {
 		log.WithError(err).WithField("email", email).Error("Error fetching senders")
 		_, _ = conn.Write([]byte("400 Error fetching senders\n"))
+		requestDurations.With(prometheus.Labels{"handler": "senders", "status": "error"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
 	if len(senders) == 0 {
 		_, _ = conn.Write([]byte("500 NO%20RESULT\n"))
+		requestDurations.With(prometheus.Labels{"handler": "senders", "status": "success"}).Observe(time.Since(now).Seconds())
 		return
 	}
 
 	_, _ = conn.Write([]byte(fmt.Sprintf("200 %s \n", strings.Join(senders, ","))))
+	requestDurations.With(prometheus.Labels{"handler": "senders", "status": "success"}).Observe(time.Since(now).Seconds())
 }
 
 // payload reads the data from the connection. It checks for valid

config.go

@@ -25,6 +25,9 @@ type Config struct {
 
 	// SendersListenAddr is the address to listen for senders requests.
 	SendersListenAddr string
+
+	// MetricsListenAddr is the address to listen for metrics requests.
+	MetricsListenAddr string
 }
 
 // NewConfig creates a new Config with default values.
@@ -81,12 +84,18 @@ func NewConfig() *Config {
 		sendersListenAddr = ":10004"
 	}
 
+	metricsListenAddr := os.Getenv("METRICS_LISTEN_ADDR")
+	if metricsListenAddr == "" {
+		metricsListenAddr = ":10005"
+	}
+
 	return &Config{
 		UserliBaseURL:     userliBaseURL,
 		UserliToken:       userliToken,
 		AliasListenAddr:   aliasListenAddr,
 		DomainListenAddr:  domainListenAddr,
 		MailboxListenAddr: mailboxListenAddr,
 		SendersListenAddr: sendersListenAddr,
+		MetricsListenAddr: metricsListenAddr,
 	}
 }

config_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"io"
 	"os"
 	"testing"
 
@@ -13,6 +14,10 @@ type ConfigTestSuite struct {
 	suite.Suite
 }
 
+func (s *ConfigTestSuite) SetupTest() {
+	log.SetOutput(io.Discard)
+}
+
 func (s *ConfigTestSuite) TestNewConfig() {
 	s.Run("fail when userli token not set", func() {
 		defer func() { log.StandardLogger().ExitFunc = nil }()
@@ -35,6 +40,7 @@ func (s *ConfigTestSuite) TestNewConfig() {
 		s.Equal(":10002", config.DomainListenAddr)
 		s.Equal(":10003", config.MailboxListenAddr)
 		s.Equal(":10004", config.SendersListenAddr)
+		s.Equal(":10005", config.MetricsListenAddr)
 	})
 
 	s.Run("custom config", func() {
@@ -44,6 +50,7 @@ func (s *ConfigTestSuite) TestNewConfig() {
 		os.Setenv("DOMAIN_LISTEN_ADDR", ":20002")
 		os.Setenv("MAILBOX_LISTEN_ADDR", ":20003")
 		os.Setenv("SENDERS_LISTEN_ADDR", ":20004")
+		os.Setenv("METRICS_LISTEN_ADDR", ":20005")
 
 		config := NewConfig()
 
@@ -53,6 +60,7 @@ func (s *ConfigTestSuite) TestNewConfig() {
 		s.Equal(":20002", config.DomainListenAddr)
 		s.Equal(":20003", config.MailboxListenAddr)
 		s.Equal(":20004", config.SendersListenAddr)
+		s.Equal(":20005", config.MetricsListenAddr)
 	})
 }
 

go.mod

@@ -4,15 +4,25 @@ go 1.23.1
 
 require (
 	github.com/h2non/gock v1.2.0
+	github.com/prometheus/client_golang v1.20.4
+	github.com/sirupsen/logrus v1.9.3
 	github.com/stretchr/testify v1.9.0
 )
 
 require (
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/h2non/parth v0.0.0-20190131123155-b4df798d6542 // indirect
+	github.com/klauspost/compress v1.17.9 // indirect
+	github.com/kr/text v0.2.0 // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
-	github.com/sirupsen/logrus v1.9.3 // indirect
+	github.com/prometheus/client_model v0.6.1 // indirect
+	github.com/prometheus/common v0.55.0 // indirect
+	github.com/prometheus/procfs v0.15.1 // indirect
 	github.com/stretchr/objx v0.5.2 // indirect
 	golang.org/x/sys v0.25.0 // indirect
+	google.golang.org/protobuf v1.34.2 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )

go.sum

@@ -1,14 +1,41 @@
+github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
+github.com/beorn7/perks v1.0.1/go.mod h1:G2ZrVWU2WbWT9wwq4/hrbKbnv/1ERSJQ0ibhJ6rlkpw=
+github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=
+github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
+github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/h2non/gock v1.2.0 h1:K6ol8rfrRkUOefooBC8elXoaNGYkpp7y2qcxGG6BzUE=
 github.com/h2non/gock v1.2.0/go.mod h1:tNhoxHYW2W42cYkYb1WqzdbYIieALC99kpYr7rH/BQk=
 github.com/h2non/parth v0.0.0-20190131123155-b4df798d6542 h1:2VTzZjLZBgl62/EtslCrtky5vbi9dd7HrQPQIx6wqiw=
 github.com/h2non/parth v0.0.0-20190131123155-b4df798d6542/go.mod h1:Ow0tF8D4Kplbc8s8sSb3V2oUCygFHVp8gC3Dn6U4MNI=
+github.com/klauspost/compress v1.17.9 h1:6KIumPrER1LHsvBVuDa0r5xaG0Es51mhhB9BQB2qeMA=
+github.com/klauspost/compress v1.17.9/go.mod h1:Di0epgTjJY877eYKx5yC51cX2A2Vl2ibi7bDH9ttBbw=
+github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
+github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
+github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
+github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
+github.com/kylelemons/godebug v1.1.0 h1:RPNrshWIDI6G2gRW9EHilWtl7Z6Sb1BR0xunSBf0SNc=
+github.com/kylelemons/godebug v1.1.0/go.mod h1:9/0rRGxNHcop5bhtWyNeEfOS8JIWk580+fNqagV/RAw=
+github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 h1:C3w9PqII01/Oq1c1nUAm88MOHcQC9l5mIlSMApZMrHA=
+github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822/go.mod h1:+n7T8mK8HuQTcFwEeznm/DIxMOiR9yIdICNftLE1DvQ=
 github.com/nbio/st v0.0.0-20140626010706-e9e8d9816f32 h1:W6apQkHrMkS0Muv8G/TipAy/FJl/rCYT0+EuS8+Z0z4=
 github.com/nbio/st v0.0.0-20140626010706-e9e8d9816f32/go.mod h1:9wM+0iRr9ahx58uYLpLIr5fm8diHn0JbqRycJi6w0Ms=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/prometheus/client_golang v1.20.4 h1:Tgh3Yr67PaOv/uTqloMsCEdeuFTatm5zIq5+qNN23vI=
+github.com/prometheus/client_golang v1.20.4/go.mod h1:PIEt8X02hGcP8JWbeHyeZ53Y/jReSnHgO035n//V5WE=
+github.com/prometheus/client_model v0.6.1 h1:ZKSh/rekM+n3CeS952MLRAdFwIKqeY8b62p8ais2e9E=
+github.com/prometheus/client_model v0.6.1/go.mod h1:OrxVMOVHjw3lKMa8+x6HeMGkHMQyHDk9E3jmP2AmGiY=
+github.com/prometheus/common v0.55.0 h1:KEi6DK7lXW/m7Ig5i47x0vRzuBsHuvJdi5ee6Y3G1dc=
+github.com/prometheus/common v0.55.0/go.mod h1:2SECS4xJG1kd8XF9IcM1gMX6510RAEL65zxzNImwdc8=
+github.com/prometheus/procfs v0.15.1 h1:YagwOFzUgYfKKHX6Dr+sHT7km/hxC76UB0learggepc=
+github.com/prometheus/procfs v0.15.1/go.mod h1:fB45yRUv8NstnjriLhBQLuOUt+WW4BsoGhij/e3PBqk=
+github.com/rogpeppe/go-internal v1.10.0 h1:TMyTOH3F/DB16zRVcYyreMH6GnZZrwQVAoYjRBZyWFQ=
+github.com/rogpeppe/go-internal v1.10.0/go.mod h1:UQnix2H7Ngw/k4C5ijL5+65zddjncjaFoBhdsK/akog=
 github.com/sirupsen/logrus v1.9.3 h1:dueUQJ1C2q9oE3F7wvmSGAaVtTmUizReu6fjN8uqzbQ=
 github.com/sirupsen/logrus v1.9.3/go.mod h1:naHLuLoDiP4jHNo9R0sCBMtWGeIprob74mVsIT4qYEQ=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
@@ -20,8 +47,11 @@ github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8
 golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.25.0 h1:r+8e+loiHxRqhXVl6ML1nO3l1+oFoWbnlu2Ehimmi34=
 golang.org/x/sys v0.25.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+google.golang.org/protobuf v1.34.2 h1:6xV6lTsCfpGD21XK49h7MhtcApnLqkfYgPcdHftf6hg=
+google.golang.org/protobuf v1.34.2/go.mod h1:qYOHts0dSfpeUzUFpOMr/WGzszTmLH+DiWniOlNbLDw=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c h1:Hei/4ADfdWqJk1ZMxUNpqntNwaWcugrBjAiHlqqRiVk=
+gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c/go.mod h1:JHkPIbrfpd72SG/EVd6muEfDQjcINNoR0C8j2r3qZ4Q=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

main.go

@@ -17,6 +17,8 @@ func main() {
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
+	go StartMetricsServer(ctx, config.MetricsListenAddr)
+
 	var wg sync.WaitGroup
 
 	wg.Add(4)

prometheus.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/collectors"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+
+	log "github.com/sirupsen/logrus"
+)
+
+var (
+	requestDurations = prometheus.NewHistogramVec(prometheus.HistogramOpts{
+		Name:    "userli_postfix_adapter_request_duration_seconds",
+		Help:    "Duration of requests to userli",
+		Buckets: prometheus.ExponentialBuckets(0.1, 1.5, 5.0),
+	}, []string{"handler", "status"})
+)
+
+// StartMetricsServer starts a new HTTP server for prometheus metrics.
+func StartMetricsServer(ctx context.Context, listenAddr string) {
+	registry := prometheus.NewRegistry()
+
+	registry.MustRegister(
+		collectors.NewGoCollector(),
+		requestDurations,
+	)
+
+	http.Handle("/metrics", promhttp.HandlerFor(registry, promhttp.HandlerOpts{}))
+
+	log.Info("Metrics server started on ", listenAddr)
+	log.Fatal(http.ListenAndServe(listenAddr, nil))
+}