test(spanner): test built-in metrics for unary RPCs

[olavloite]

Adds tests for built-in metrics for unary RPCs.

[gemini-code-assist]

Code Review

This pull request refactors the observability mock tests in mock_tests.rs by replacing a manual atomic counter with mockall::Sequence for sequential mock expectations. It also adds a comprehensive suite of new mock tests to verify that internal client metrics, such as attempt and operation counts, are correctly recorded for various Spanner operations (including batch DML, transaction retries, rollbacks, midstream resumptions, and partitioned reads/queries). There are no review comments, and I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.40%. Comparing base (4084332) to head (fc8424e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6505   +/-   ##
=======================================
  Coverage   96.39%   96.40%           
=======================================
  Files         301      301           
  Lines       84762    84762           
=======================================
+ Hits        81710    81716    +6     
+ Misses       3052     3046    -6     

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