impl(GCS+gRPC): format gRPC metric names

[coryan]

Part of the work for #13998

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.06%. Comparing base (52e16d5) to head (bab80b6).

Files	Patch %	Lines
.../storage/internal/grpc/metrics_exporter_options.cc	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14381      +/-   ##
==========================================
- Coverage   93.06%   93.06%   -0.01%     
==========================================
  Files        2191     2191              
  Lines      193206   193207       +1     
==========================================
- Hits       179810   179806       -4     
- Misses      13396    13401       +5     

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[dbolduc]

optional: use the returned MetricFormatterOption in a unit test on some known grpc metric name

Or factor out the function and add unit tests for it?

[coryan]

/gcbrun