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 (50557c3, aeed0b9, f4bfe4f)
• ✅ login: vlad-coman-hs / name: vlad-coman-hs (a5f9f45)

[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 81.8%. Comparing base (b45e747) to head (aeed0b9).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8191   +/-   ##
=====================================
  Coverage   81.8%   81.8%           
=====================================
  Files        194     194           
  Lines      13377   13388   +11     
=====================================
+ Hits       10951   10962   +11     
  Misses      2029    2029           
  Partials     397     397           

Files with missing lines	Coverage Δ
...entation/google.golang.org/grpc/otelgrpc/config.go	87.3% <100.0%> (+1.0%)	⬆️
...n/google.golang.org/grpc/otelgrpc/stats_handler.go	98.5% <100.0%> (+<0.1%)	⬆️

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