cleanup(gax-internal): macros use literal attribute names

[coryan]

By using the literal attribute names we can reduce the number of constants exported by google-cloud-gax-internal. And therefore reduce the number of future breaking changes.

Part of the work for #4772

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.01%. Comparing base (388da11) to head (02f5eaf).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/observability/client_signals/with_client_span.rs	93.75%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5209      +/-   ##
==========================================
- Coverage   98.02%   98.01%   -0.01%     
==========================================
  Files         214      214              
  Lines       44218    44250      +32     
==========================================
+ Hits        43344    43371      +27     
- Misses        874      879       +5     

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