Split metrics into data metrics and server metrics

[drmingdrmer]

RaftMetrics encapsulates a range of information about the internal states of
a Raft node, including the server state (e.g., Leader, Follower), and
data-related states such as the last log index and the last applied log index.

Applications interested in monitoring these details can subscribe to
RaftMetrics to receive updates when these metrics change, with
Raft::metrics().

However, some metrics, like last_log_index, are frequently updated, while
others, such as current_term and state, change less often.

For applications solely concerned with server state transitions (like switching
from Leader to Follower), being notified of changes to last_log_index can
result in unnecessary wake-ups.

To optimize this, it would be beneficial to add two subsets of the metrics:

• Metrics that are not data-related, such as current_term, vote, and state.
• Metrics that are data-related, including last_log_index and last_applied.

By segregating these metrics, applications can subscribe to the relevant subset
and reduce the overhead of processing frequent updates that are not pertinent to
their needs.

TODO:

• add Raft::data_metrics() to return a watch::Receiver<RaftDataMetrics>
  that contains only metrics about log, state machine and snapshot.

• add Raft::server_metrics() to return a
  watch::Receiver<RaftServerMetrics> that contains only server related
  metrics, such as vote, leader, id, running_state, membership

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[schreter]

BTW, I was about to suggest the same :-).

[YangKian]

Hi, may I claim this assignment? I'd like to have some try

[drmingdrmer]

Hi, may I claim this assignment? I'd like to have some try

Sure. Thank you!

Please keep current API and data types and just add new APIs. :)

[YangKian]

Please keep current API and data types and just add new APIs. :)

get it~