impl(bigtable): test grpc otel plugin configuration

[scotthart]

This PR factors out the configuration into a separate function that we can more easily test.

[gemini-code-assist]

Code Review

This pull request refactors the gRPC metrics exporter by extracting the configuration logic of the OpenTelemetry plugin into a new GrpcMetricsPluginConfig struct and helper functions (MakeGrpcMetricsPluginConfig). It also adds corresponding unit tests to validate the configuration arguments. The feedback suggests passing the opentelemetry::sdk::resource::Resource object by value instead of by const reference in MakeGrpcMetricsPluginConfig to avoid unnecessary copies and allow moving it into the lambda function.

[codecov]

Codecov Report

❌ Patch coverage is 91.80328% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.24%. Comparing base (acf90c3) to head (26153dd).

Files with missing lines	Patch %	Lines
...ud/bigtable/internal/grpc_metrics_exporter_test.cc	86.11%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16347   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files        2227     2227           
  Lines      209543   209594   +51     
=======================================
+ Hits       193284   193337   +53     
+ Misses      16259    16257    -2     

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