chore(spanner): remove allow(dead_code) annotations

[olavloite]

Remove several obsolete allow(dead_code) annotations from the Spanner client.

[gemini-code-assist]

Code Review

This pull request cleans up unused #[allow(dead_code)] attributes and outdated TODO comments across several files in the spanner crate, including client.rs, key.rs, mutation.rs, builder.rs, stream.rs, and types.rs. Additionally, it adds a targeted #[allow(dead_code)] attribute and a TODO comment to the headers field and method in BatchWriteStream to prepare for future metrics and header recording. 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 96.39%. Comparing base (4084332) to head (ce60f84).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6506      +/-   ##
==========================================
- Coverage   96.39%   96.39%   -0.01%     
==========================================
  Files         301      301              
  Lines       84762    84762              
==========================================
- Hits        81710    81703       -7     
- Misses       3052     3059       +7     

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