otelgrpc: add MetricAttributesFn option to allow adding dynamic attributes on metrics

[vlad-coman-hs]

This PR addresses the need of adding dynamic attributes on the auto-instrumented metrics from otelgrpc. It potentially addresses #6026 without giving access to the payload since it is not available during every phase which adds complexity when trying to add the labels to all available metrics. It might also generate more confusion to the end user by exposing internal implementation details.

Initial draft that implemented the Labeler pattern from otelhttp (it also explains why it was closed): #8135

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: vlad-coman-hs / name: Vlad Coman (baaae0f)
• ✅ login: vlad-coman-hs / name: vlad-coman-hs (6e56dbb)

[dashpole]

Can you add documentation or an example_test.go to demonstrate how someone could use this to add attributes to metrics?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.3%. Comparing base (9b21944) to head (6e56dbb).
⚠️ Report is 32 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8191   +/-   ##
=====================================
  Coverage   82.3%   82.3%           
=====================================
  Files        193     193           
  Lines      13769   13779   +10     
=====================================
+ Hits       11334   11344   +10     
  Misses      2030    2030           
  Partials     405     405           

Files with missing lines	Coverage Δ
...entation/google.golang.org/grpc/otelgrpc/config.go	87.0% <100.0%> (+0.8%)	⬆️
...n/google.golang.org/grpc/otelgrpc/stats_handler.go	98.5% <100.0%> (+<0.1%)	⬆️

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[vlad-coman-hs]

Hello @dashpole,
Just wanted to check in on this PR. Please let me know if there's anything I should adjust or clarify.
Thank you for your time!

[dashpole]

There was a release, so you need to move the changelog entry up to the unreleased section. You may need to add the appropriate header if the section doesn't already exist.

[vlad-coman-hs]

Hi! Thanks for reviewing this. Please let me know if there's anything else needed from my side before merging.

[dmathieu]

Nothing needed on your end for now. We require a second approval though.

[flc1125]

Based on the current situation, I think there is no problem; however, I am not entirely sure if this is the best implementation plan.