Metrics doc

[iGxnon]

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

• Change quick start

• Fix benchmark

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 56.41026% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 75.49%. Comparing base (e35b35a) to head (239b85c). Report is 84 commits behind head on master.

Files	Patch %	Lines
crates/curp/src/server/metrics.rs	62.50%	12 Missing :warning:
crates/curp/src/server/raw_curp/mod.rs	0.00%	3 Missing :warning:
crates/curp/src/server/curp_node.rs	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #685 +/- ## ========================================== - Coverage 75.55% 75.49% -0.06% ========================================== Files 180 186 +6 Lines 26938 27478 +540 Branches 26938 27478 +540 ========================================== + Hits 20353 20745 +392 - Misses 5366 5458 +92 - Partials 1219 1275 +56 ```

:umbrella: View full report in Codecov by Sentry.
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[iGxnon]

/cc @bsbds Could you take a look?

[mergify[bot]]

@iGxnon Convert your pr to draft since CI failed

[Phoenix500526]

@Mergifyio rebase

[mergify[bot]]

rebase

✅ Branch has been successfully rebased

[mergify[bot]]

@iGxnon Convert your pr to draft since CI failed

[iGxnon]

rebased manually

[mergify[bot]]

@iGxnon Your PR is in conflict and cannot be merged.