[SDK] Use kAllW3CTraceContext2Flags

[saphal1998]

Fixes #2283

Changes

• Enable W3C Trace Context v2 trace-flag masking in the tracer so the random trace-id flag is preserved.
• Add test coverage to verify the random flag is handled correctly across trace flags, HTTP trace context propagation, span creation, and OTLP export.

NOTE - Outside of the TraceFlagsTest, I did ask an LLM to determine what other surfaces the random bit would manifest, and the test cases added were suggestions, to me personally, these surfaces made sense, but I could be missing critical tests that I need to add or perhaps these tests are redundant, so please let me know!

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 82.06%. Comparing base (dbf6567) to head (44f9bfc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4012      +/-   ##
==========================================
+ Coverage   82.06%   82.06%   +0.01%     
==========================================
  Files         385      385              
  Lines       15891    15892       +1     
==========================================
+ Hits        13039    13040       +1     
  Misses       2852     2852              

Files with missing lines	Coverage Δ
api/include/opentelemetry/trace/trace_flags.h	100.00% <ø> (ø)
sdk/src/trace/tracer.cc	86.12% <ø> (ø)

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[saphal1998]

@marcalff I'm a little unsure as to why checks that fail, actually fail, could you perhaps point me to what might have triggered these failures? thank you for the help!

[marcalff]

@marcalff I'm a little unsure as to why checks that fail, actually fail, could you perhaps point me to what might have triggered these failures? thank you for the help!

From the IWYU CI logs:

include-what-you-use reported 1 warning(s) exceeding the existing warning limit of 0 with cmake options preset 'all-options-abiv1-preview'

This is only a summary.

In this case, see the section iwyu_tool in the logs, to see the actual errors.

Warning: include-what-you-use reported diagnostics:

/home/runner/work/opentelemetry-cpp/opentelemetry-cpp/sdk/test/trace/tracer_test.cc should add these lines:
#include "opentelemetry/trace/trace_flags.h"

Note that the build continues on failures (make -k), to report as many as possible in one run, which is why we have to search for each issue then.

[marcalff]

@saphal1998

The magic text to look for in logs, in the iwyu_tool section, is:

Warning: include-what-you-use reported diagnostics:

[marcalff]

@saphal1998

For the build, please fix include-what-you-use as indicated earlier.

For testing, see the existing CI:

  w3c_trace_context_compliance_v1:
    name: W3C Distributed Tracing Validation V1

and add the same for level 2 validation.

See also the following directory for tooling: ext/test/w3c_tracecontext_http_test_server.

[marcalff]

This is the root cause of the new clang-tidy warning, which breaks CI.

This header file includes itself, remove.