soroban-rpc: Add /metrics endpoint

[tamirms]

What

Add /metrics prometheus endpoint with the following set of metrics:

• DB (similar to horizon)

  • connections
    • open
    • in-use
    • waited-for
    • wait-duration
  • queries per-method
    • mean duration
    • count

• General

  • Version running
  • Uptime (comes out of the box w prometheus, I think)
  • Golang stuff like goroutines etc

• Logging

  • Lines/entries logged (with severity)

Why

#496

This PR does not provide all the metrics specified in the issue above. The remaining metrics be included in separate upcoming PRs.

Known limitations

[N/A]

[tsachiherman]

Tamir, I don't believe that the PrometheusRegistry should be part of the local configuration.
( unless you can convince me otherwise ).
I think that it should be a member of the daemon.

The local config is for entries that we would load from a configuration file. I don't believe the registry is part of that..

[tsachiherman]

Could you please add a unit test to confirm that this PR work ?

[tamirms]

@tsachiherman can you take another look? I think I've addressed your comments

[tsachiherman]

Tamir, my intent in asking for a unit test was to have a http-client unit test that would verify that the metrics endpoint works as expected.

I don't have any reason to believe that it wouldn't, and I don't want to block you from merging this one in - but I do want to have ( eventually ) a concrete unit test to cover all the http endpoints.

[tamirms]

@tsachiherman could you take another look? I believe I've addressed your feedback.