chore(spanner): trace server streaming queries and reads in ResultSet

[olavloite]

Trace ExecuteStreamingSql and StreamingRead operation latencies across stream initialization and first result delivery.

[gemini-code-assist]

Code Review

This pull request introduces built-in metrics and observability support for Spanner streaming operations, such as streaming SQL and streaming reads, by propagating an Observability instance and method names through transaction and result set parameters. The feedback highlights a critical issue where the disabled Observability instance from the inner Spanner client (client.spanner.o11y) is used instead of the active one from the DatabaseClient (client.o11y), which would prevent metrics from being recorded.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.08%. Comparing base (c5f374b) to head (d256714).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6204      +/-   ##
==========================================
+ Coverage   96.04%   96.08%   +0.04%     
==========================================
  Files         269      269              
  Lines       68390    68440      +50     
==========================================
+ Hits        65686    65763      +77     
+ Misses       2704     2677      -27     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request integrates observability and metrics tracking into Spanner's streaming operations. Specifically, it updates ResultSetParams to include o11y (observability) and method_name fields, allowing streaming SQL and read operations to trace their execution. The Observability struct was updated to be cloneable by wrapping its internal metrics and provider in Arc, and comprehensive unit tests were added to verify the new observability features. There are no review comments, and we have no feedback to provide.