feat(o11y): remove google-cloud-unstable-tracing guards

[haphungw]

Remove google_cloud_unstable_tracing feature guards from handwritten code (src/gax-internal/, src/storage/, and tests), making observability features unconditionally available.

This PR is the first step in resolving a cross-repository dependency involving the code generator (librarian/sidekick) googleapis/librarian#5343:
Since we updated the generator to produce code without guards before removing the guards manually in gax-internal, the resulting generated code failed compilation checks because it attempted to use features that are still feature-gated in the library.

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Correct Steps:
Manual removal in Rust (This PR).
Librarian change to remove guard generation googleapis/librarian#5103.
Regeneration PR in Rust #5292.

Note: We also kept cfg(google_cloud_unstable_tracing) in Cargo.toml for now, so that the existing generated code still in the repo doesn't fail the unexpected_cfgs lint. This should be removed in the regeneration PR for google-cloud-rust.

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Fixes googleapis/librarian#5343

[codecov]

Codecov Report

❌ Patch coverage is 96.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.76%. Comparing base (5002c76) to head (4096f6e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/storage/src/storage/transport.rs	91.48%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5382      +/-   ##
==========================================
+ Coverage   97.59%   97.76%   +0.17%     
==========================================
  Files         222      222              
  Lines       46609    46599      -10     
==========================================
+ Hits        45486    45556      +70     
+ Misses       1123     1043      -80     

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[haphungw]

This approach couldn't solve the circular dependency problem. Closing.