fix: ignore deferred spans when delegating sampling decision

[lbeschastny]

Propagators could leave spans in a deferred state. For example, the following valid set of b3-multi headers does eactly that:

X-B3-TraceId: 68f8e1942794b9a8072bfc5f4f2c368f
X-B3-SpanId: a4295738dcb1aac4

but ShouldSample implementation for opentelemetry_sdk::trace::sampler::Sampler only checks that the parent span is active.

The correct behaviour would be to fall back to delegate sampler when the parent spans has deferred flag set by a propagator.

Changes

Sampler::ParentBased branch in the should_sample method now checks is the parent span has TRACE_FLAG_DEFERRED set and filters all deferred spans.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: lbeschastny / name: Leonid Beschasny (cb7930d)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.9%. Comparing base (d721037) to head (cb7930d).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3209   +/-   ##
=====================================
  Coverage   80.8%   80.9%           
=====================================
  Files        128     128           
  Lines      23090   23118   +28     
=====================================
+ Hits       18676   18704   +28     
  Misses      4414    4414           

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