Allow setting per-request custom metrics in gRPC stats handler.

[dhowden]

Updates #6026, similar to #5876 but for gRPC.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: dhowden / name: David Howden (bdb739e, 02f9e15, b474ced)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 65.7%. Comparing base (8a49875) to head (b474ced).

Files with missing lines	Patch %	Lines
...entation/google.golang.org/grpc/otelgrpc/config.go	0.0%	5 Missing ⚠️
...n/google.golang.org/grpc/otelgrpc/stats_handler.go	0.0%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6092     +/-   ##
=======================================
- Coverage   65.7%   65.7%   -0.1%     
=======================================
  Files        203     203             
  Lines      13037   13044      +7     
=======================================
+ Hits        8568    8570      +2     
- Misses      4206    4212      +6     
+ Partials     263     262      -1     

Files with missing lines	Coverage Δ
...n/google.golang.org/grpc/otelgrpc/stats_handler.go	96.6% <0.0%> (-3.4%)	⬇️
...entation/google.golang.org/grpc/otelgrpc/config.go	85.8% <0.0%> (-4.3%)	⬇️

... and 1 file with indirect coverage changes