feat(trace): support the W3C random trace ID flag

[ayush-that]

Fixes #3270

Changes

Random trace IDs can now wear their little badge.

Adds RANDOM helpers, generator opt-in, SDK flag handling, and W3C propagation. Custom generators must implement is_random() explicitly; RandomIdGenerator returns true. Children retain the flag even when dropped.

B3's private deferred bit moves out of the way. Local drops clear it, and OTLP conversions filter private bits while preserving W3C flags.

Tests, Clippy, formatting, docs, and all precommit stages passed.

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)

[codecov]

Codecov Report

❌ Patch coverage is 99.77827% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.6%. Comparing base (02b2f15) to head (b3247c3).

Files with missing lines	Patch %	Lines
opentelemetry-appender-tracing/src/layer.rs	50.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3718     +/-   ##
=======================================
+ Coverage   84.2%   84.6%   +0.3%     
=======================================
  Files        123     123             
  Lines      30484   30894    +410     
=======================================
+ Hits       25686   26144    +458     
+ Misses      4798    4750     -48     

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[opentelemetry-pr-dashboard]

Pull request dashboard status

Waiting on maintainers · refreshed 2026-10-17 01:45 UTC

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[lalitb]

LGTM. Although the SDK randomness extension is still marked Development in the spec, Python and Java already expose it in their stable SDKs, so I’m comfortable following the same approach here, unless other @open-telemetry/rust-maintainers / @open-telemetry/rust-approvers have any concerns.

[scottgerring]

hey @ayush-that , thanks for addressing comments! Can you resolve the conflict with main so we can merge this?