feat(o11y): add record_polling_attributes macro

[haphungw]

Introduce the record_polling_attributes! macro to google-cloud-lro to encapsulate the boilerplate required for injecting LRO telemetry attributes (e.g., poll attempt count) into the active span.

For googleapis/librarian#6275

[gemini-code-assist]

Code Review

This pull request introduces a new macro, record_polling_attributes, to inject LRO-specific telemetry attributes into the active span. The review feedback points out that the macro evaluates the $span expression twice when the google_cloud_unstable_tracing feature is enabled, which could lead to side effects, and ignores it when disabled, potentially triggering unused variable warnings. A refactoring is suggested to bind $span to a local reference and handle the disabled case safely.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.89%. Comparing base (1059b90) to head (c6c4d38).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5838   +/-   ##
=======================================
  Coverage   97.89%   97.89%           
=======================================
  Files         226      226           
  Lines       57713    57713           
=======================================
+ Hits        56496    56497    +1     
+ Misses       1217     1216    -1     

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