feat(o11y): remove google_cloud_unstable_tracing gate for LRO tracing

[haphungw]

No description provided.

[gemini-code-assist]

Code Review

This pull request stabilizes tracing and poller options across various generated Google Cloud clients by removing the google_cloud_unstable_tracing conditional compilation flags. It also exposes internal observability error types and helper functions by increasing their visibility to public. Regarding the feedback, a naming convention issue was identified in observability/errors.rs where a method returning an owned String was prefixed with as_ instead of to_.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.85%. Comparing base (737706a) to head (93df64f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5897      +/-   ##
==========================================
- Coverage   97.90%   97.85%   -0.05%     
==========================================
  Files         234      236       +2     
  Lines       59451    60067     +616     
==========================================
+ Hits        58204    58780     +576     
- Misses       1247     1287      +40     

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