chore(spanner): configure built-in metrics reader

[olavloite]

• Configure the built-in metrics reader to export once every 60s
• Set up bucket boundaries
• Disable built-in metrics when connected to the emulator
• Harden the parsing of server timings

[gemini-code-assist]

Code Review

This pull request enhances the Spanner observability and metrics module by adding support to disable metrics when using the emulator, defining custom bucket boundaries for latency histograms, and setting a default 60-second export interval. It also refactors server timing parsing for better validation and adds comprehensive unit tests to verify these changes. I have no feedback to provide.

[codecov]

Codecov Report

❌ Patch coverage is 98.78049% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.10%. Comparing base (57dae51) to head (4d8c2f0).

Files with missing lines	Patch %	Lines
src/spanner/src/observability/exporter.rs	66.66%	1 Missing ⚠️
src/spanner/src/observability/metrics.rs	99.37%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6242      +/-   ##
==========================================
+ Coverage   96.05%   96.10%   +0.04%     
==========================================
  Files         271      271              
  Lines       68537    68681     +144     
==========================================
+ Hits        65835    66004     +169     
+ Misses       2702     2677      -25     

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