[dbarker]

Contributes to #4711

The current Tracer::StartSpan API and SDK disabled "noop behavior" is to return a non-recording span with an invalid span context, however the spec requires the noop span to contain the parent's context if valid. This is important to prevent breaking traces that propagate through libraries/processes where tracers or the entire SDK is disabled.

The spec defines the noop behavior here: https://opentelemetry.io/docs/specs/otel/trace/api/#behavior-of-the-api-in-the-absence-of-an-installed-sdk

The API MUST return a non-recording Span with the SpanContext in the parent Context (whether explicitly given or implicit current).

This PR only addresses the SDK Tracer's disabled behavior to meet the spec. A follow up PR will be required to fix the API's noop tracer to meet the spec for when the SDK is fully disabled.

Changes

• Update SDK Tracer to return a non-recording span with the parent's span context when disabled.
• Add test cases for all parent context propagation options when calling StartSpan
• Add new tracer benchmark cases to cover the new behavior

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.81%. Comparing base (a273994) to head (3b3ac18).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4710      +/-   ##
==========================================
+ Coverage   86.81%   86.81%   +0.01%     
==========================================
  Files         525      525              
  Lines       20739    20741       +2     
==========================================
+ Hits        18003    18005       +2     
  Misses       2736     2736              

Files with missing lines	Coverage Δ
sdk/src/trace/tracer.cc	84.15% <100.00%> (+0.40%)	⬆️

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[marcalff]

LGTM, thanks for the fix.