[API] reduce allocation in no span case

[proost]

Changes

Comment: #4191 (comment)

Reducing allocation in the no span case. Because a new default span return nostd::shared_ptr<Span>(new DefaultSpan(SpanContext::GetInvalid())); is created when we get context form GetSpan.

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

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.15%. Comparing base (0e2406e) to head (e4cfad2).

Files with missing lines	Patch %	Lines
api/include/opentelemetry/trace/context.h	63.64%	4 Missing ⚠️
...pentelemetry/sdk/metrics/exemplar/reservoir_cell.h	66.67%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4254      +/-   ##
==========================================
- Coverage   78.15%   78.15%   -0.00%     
==========================================
  Files         440      440              
  Lines       18741    18736       -5     
==========================================
- Hits        14645    14641       -4     
+ Misses       4096     4095       -1     

Files with missing lines	Coverage Δ
...de/opentelemetry/trace/propagation/b3_propagator.h	93.66% <100.00%> (ø)
...entelemetry/trace/propagation/http_trace_context.h	92.86% <100.00%> (ø)
...i/include/opentelemetry/trace/propagation/jaeger.h	97.83% <100.00%> (ø)
sdk/src/logs/logger.cc	94.12% <100.00%> (+3.61%)	⬆️
sdk/src/trace/tracer.cc	87.50% <100.00%> (ø)
...pentelemetry/sdk/metrics/exemplar/reservoir_cell.h	56.53% <66.67%> (-1.81%)	⬇️
api/include/opentelemetry/trace/context.h	82.61% <63.64%> (-17.39%)	⬇️

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[dbarker]

Looks good to me. Thanks for the PR!

It needs additional review and approval as this is an expansion of the API.

[dbarker]

Not an ABI break. No observable behavior change.

[marcalff]

LGTM, thanks for the fix.

[marcalff]

Do not merge:

Release 1.28.0 needs to be published first, and this PR can be merged after.

This is to limit last minute changes to API just when releasing.

[ThomsonTan]

Could the active variant alternative contain an empty nostd::shared_ptr<Span> here? nostd::get_if would still return a non-null pointer to the stored shared_ptr, so this condition would succeed even though *maybe_span is null, and the next line would dereference it.

The same issue applies to maybe_span_context below. Should both branches also check that the stored shared_ptr is non-null and return SpanContext::GetInvalid() otherwise?