impl(bigtable): add benchmark command line arg to set metrics period

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a new --metrics-period command-line option to the Bigtable benchmarks, allowing users to configure the frequency at which client-side metrics are exported. This includes parsing the option as a duration, validating that it is positive, and applying it to the connection options via MetricsPeriodOption. Relevant unit tests have also been added to verify the parsing, validation, and table creation behavior. The feedback suggests removing the redundant bigtable:: namespace prefix when setting the MetricsPeriodOption to maintain consistency with existing patterns in the file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.23%. Comparing base (154f9ef) to head (43c4a89).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16338   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files        2227     2227           
  Lines      209283   209312   +29     
=======================================
+ Hits       193026   193058   +32     
+ Misses      16257    16254    -3     

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