impl(gax-internal): Define OpenTelemetry key constants for HttpSpanInfo

[westarle]

for #3239

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.19%. Comparing base (576ab46) to head (4c5d278).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3367      +/-   ##
==========================================
- Coverage   96.21%   96.19%   -0.03%     
==========================================
  Files         115      115              
  Lines        4574     4574              
==========================================
- Hits         4401     4400       -1     
- Misses        173      174       +1     

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[coryan]

And I would avoid the passive voice, and format these differently:

Suggested change

	/// Span Name for OpenTelemetry interop. Recommended to be "{http.request.method} {url.template}" if url.template is available, otherwise "{http.request.method}".
	/// Span Name for OpenTelemetry interop.
	///
	/// If `url.template` is available use "{http.request.method} {url.template}", otherwise use "{http.request.method}".

[westarle]

Thank you, I updated all the docs, let me know if it's overkill.

[coryan]

Before I forget, should we be using:

https://docs.rs/opentelemetry-semantic-conventions/latest/opentelemetry_semantic_conventions/trace/index.html