fix(gax-internal): keep traces as Unset on success

[coryan]

The convention / rules for OpenTelemetry are to keep traces as Unset on success.

Fixes #4938. See that bug for background and justification, because we all think it is not intuitive.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.94%. Comparing base (15d5fce) to head (6daec9c).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/gax-internal/src/observability/client_tracing.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4967      +/-   ##
==========================================
- Coverage   93.94%   93.94%   -0.01%     
==========================================
  Files         221      221              
  Lines        8538     8536       -2     
==========================================
- Hits         8021     8019       -2     
  Misses        517      517              

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