impl(GCS+gRPC): always enable gRPC metrics

[coryan]

Part of the work for #13998

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This change is 

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 25.37313% with 50 lines in your changes are missing coverage. Please review.

Project coverage is 93.79%. Comparing base (1db79c6) to head (3cfea25).

Files	Patch %	Lines
...oud/storage/internal/grpc/metrics_exporter_impl.cc	0.00%	45 Missing :warning:
...ogle/cloud/storage/internal/grpc/enable_metrics.cc	44.44%	5 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14259      +/-   ##
==========================================
+ Coverage   93.26%   93.79%   +0.52%     
==========================================
  Files        2213     2301      +88     
  Lines      193275   204108   +10833     
==========================================
+ Hits       180255   191438   +11183     
+ Misses      13020    12670     -350     

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