feat(otel): capture gRPC response metadata in traces

[dbolduc]

Fixes #13275

Capture response metadata over gRPC in traces.

Give special handling to binary headers. I am open to other suggestions on how they should be displayed.

For the tests - note that ValidatePropagator() calls ValidateMetadataFixture::GetMetadata() under the hood, so it both adds a layer of testing and prevents crashes when trying to access the server metadata stored in the grpc::ClientContext.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (888b7dd) 93.06% compared to head (91724ce) 93.06%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13278   +/-   ##
=======================================
  Coverage   93.06%   93.06%           
=======================================
  Files        2137     2137           
  Lines      186610   186640   +30     
=======================================
+ Hits       173666   173698   +32     
+ Misses      12944    12942    -2     

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