[CODE HEALTH] Cleanup nostd variant access for noexcept methods in api and sdk

[dbarker]

Fixes # (issue)

Changes

• replace nostd::holds_alternative/nostd::get pattern with nostd::get_if in api and sdk.
• unit test for trace/context.h
• expanded test cases for sdk/logger to cover trace metadata
• fixes the following warnings

---

File	Line	Message
api/include/opentelemetry/nostd/function_ref.h	81	forwarding reference parameter 'f' is never forwarded inside the function body
api/include/opentelemetry/trace/context.h	26	an exception may be thrown in function 'IsRootSpan' which should not throw exceptions
sdk/src/logs/logger.cc	59	an exception may be thrown in function 'CreateLogRecord' which should not throw exceptions

---

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 90.23%. Comparing base (0b1c704) to head (81c5775).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3965      +/-   ##
==========================================
+ Coverage   90.13%   90.23%   +0.11%     
==========================================
  Files         230      230              
  Lines        7293     7286       -7     
==========================================
+ Hits         6573     6574       +1     
+ Misses        720      712       -8     

Files with missing lines	Coverage Δ
...pi/include/opentelemetry/baggage/baggage_context.h	100.00% <100.00%> (ø)
api/include/opentelemetry/nostd/function_ref.h	77.78% <ø> (ø)
api/include/opentelemetry/trace/context.h	100.00% <100.00%> (+8.34%)	⬆️
api/include/opentelemetry/trace/tracer.h	100.00% <100.00%> (ø)
sdk/src/logs/logger.cc	97.06% <100.00%> (+14.84%)	⬆️
sdk/src/trace/tracer.cc	84.85% <100.00%> (-0.22%)	⬇️

... and 1 file with indirect coverage changes

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