fix(otel): avoid infinite trace export loop

[dbolduc]

Fixes #14611

Avoid an infinite export loop when GOOGLE_CLOUD_CPP_OPENTELEMETRY_TRACING is set by never tracing the Cloud Trace client.

While the solution is not good, it seemed less bad than the alternatives listed in the parent issue.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.59%. Comparing base (36c3aee) to head (8150ba7).
Report is 2 commits behind head on main.

Files	Patch %	Lines
generator/internal/option_defaults_generator.cc	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14612      +/-   ##
==========================================
- Coverage   93.59%   93.59%   -0.01%     
==========================================
  Files        2316     2316              
  Lines      207108   207115       +7     
==========================================
- Hits       193846   193841       -5     
- Misses      13262    13274      +12     

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[scotthart]

Reviewed 2 of 3 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dbolduc)