chore(spanner): implement shutdown and Drop handler for Observability meter provider

[olavloite]

• Add explicit Observability::shutdown(&self) method to shut down the underlying OpenTelemetry SdkMeterProvider, flushing pending metric data points and stopping periodic reader background tasks.
• Implement Drop for Observability so telemetry resources are cleanly released when clients are closed or dropped.
• Ignore OTelSdkError::AlreadyShutdown on repeated calls to shutdown(&self) to prevent false-positive warning logs.

[gemini-code-assist]

Code Review

This pull request implements graceful shutdown and resource cleanup for the OpenTelemetry SdkMeterProvider in the Spanner observability module. It renames _meter_provider to meter_provider, implements a shutdown method that handles AlreadyShutdown errors, and implements the Drop trait to automatically trigger shutdown when Observability is dropped. Additionally, new unit tests are added to verify the shutdown and drop behaviors. I have no feedback to provide as the changes are well-implemented and include appropriate test coverage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.07%. Comparing base (93d9a0f) to head (d900134).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6246   +/-   ##
=======================================
  Coverage   96.07%   96.07%           
=======================================
  Files         273      273           
  Lines       68742    68766   +24     
=======================================
+ Hits        66042    66066   +24     
  Misses       2700     2700           

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