Update 240-metrics.mdx - let -> const

There was no good reason to use `let` that I could think of here 🤔

content/200-concepts/100-components/02-prisma-client/240-metrics.mdx

@@ -82,7 +82,7 @@ When you retrieve metrics in JSON format, you can use them in the format they ar
 To retrieve metrics in JSON format, add the following lines to your application code:
 
 ```ts
-let metrics = await prisma.$metrics.json()
+const metrics = await prisma.$metrics.json()
 console.log(metrics)
 ```
 
@@ -343,7 +343,7 @@ When you retrieve Prisma metrics in Prometheus format, you can use them in the f
 To retrieve metrics in Prometheus format, add the following lines to your application code:
 
 ```ts
-let metrics = await prisma.$metrics.prometheus()
+const metrics = await prisma.$metrics.prometheus()
 console.log(metrics)
 ```
 
@@ -485,7 +485,7 @@ const port = 4000
 const prisma = new PrismaClient()
 
 app.get('/metrics', async (_req, res: Response) => {
-  let metrics = await prisma.$metrics.prometheus()
+  const metrics = await prisma.$metrics.prometheus()
   res.end(metrics)
 })
 
@@ -509,8 +509,8 @@ const register = new prom.Registry()
 prom.collectDefaultMetrics({ register })
 
 app.get('/metrics', async (_req, res: Response) => {
-  let prismaMetrics = await prisma.$metrics.prometheus()
-  let appMetrics = await register.metrics()
+  const prismaMetrics = await prisma.$metrics.prometheus()
+  const appMetrics = await register.metrics()
   res.end(prismaMetrics + appMetrics)
 })
 
@@ -528,7 +528,7 @@ Global labels work with JSON and Prometheus-formatted metrics.
 For example, to add global labels to JSON-format metrics, add the following code to your application:
 
 ```ts
-let metrics = prisma.$metrics.json({
+const metrics = prisma.$metrics.json({
   globalLabels: { server: 'us_server1', app_version: 'one' },
 })
 console.log(metrics)