Add ipa-metrics crate

[akoshelev]

Follows the design described here https://docs.google.com/document/d/1cRnE024bi7KJYeqMT8yMYluIplA8992bNIaeKBmrXp4/edit#heading=h.d8g3vju6yaqd

I have a working solution where this crate is integrated with IPA. There will be 3 separate PRs to make the transition

• [This PR] ipa-metrics crate with core functionality
• ipa-metrics-tracing that adds support for metric partitioning in tests
• Migrate IPA to new metrics engine

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.75746% with 25 lines in your changes missing coverage. Please review.

Project coverage is 93.59%. Comparing base (57fa8a5) to head (7331a1c). Report is 52 commits behind head on main.

Files with missing lines	Patch %	Lines
ipa-metrics/src/label.rs	85.88%	12 Missing :warning:
ipa-metrics/src/context.rs	89.65%	9 Missing :warning:
ipa-metrics/src/collector.rs	96.19%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1356 +/- ## ========================================== + Coverage 93.49% 93.59% +0.10% ========================================== Files 210 221 +11 Lines 34925 36522 +1597 ========================================== + Hits 32652 34183 +1531 - Misses 2273 2339 +66 ```

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[akoshelev]

I don't see the periodic flushing to the collector in here, is that something that needs to be done as part of the integration?

Yea I decided not to put it in this iteration. It both simplifies the reviews and I don't see a strong need for that just yet. Test runs on draft shown that metrics get updated fairly frequently. It is possible to add it later without changing the API I think