[configgrpc] Insert headers automatically

[jade-guiton-dd]

Description

This PR uses the grpc.WithUnaryInterceptor option to add the metadata configured in ClientConfig.Headers automatically to requests made with the created grpc.ClientConn. Currently, users of configgrpc must extract the headers from the config and insert them manually in each request.

The current implementation is meant to be backward-compatible with code that manually inserts the headers: if a header key is already present in the Context passed to the interceptor, it will not be modified and no duplicate will be added.

Link to tracking issue

Fixes #12307

Testing

The tests in otlpexporter check that headers are properly inserted, and they still pass after removing the OTLP-specific headers code. I also added a new test in configgrpc for testing unary RPC calls.

I'm not sure how to easily test streaming RPC calls however (hence the lacking code coverage), but since the docs imply that adding metadata to streaming RPC calls is the same as unary calls, it should hopefully work.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.38%. Comparing base (765e4a5) to head (27f487f).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
config/configgrpc/configgrpc.go	82.35%	3 Missing ⚠️

❌ Your patch status has failed because the patch coverage (85.71%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12320      +/-   ##
==========================================
- Coverage   91.39%   91.38%   -0.02%     
==========================================
  Files         468      468              
  Lines       25591    25610      +19     
==========================================
+ Hits        23390    23404      +14     
- Misses       1785     1790       +5     
  Partials      416      416              

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