sdk: expose Tracer::id_generator()

[djc]

Changes

#1755 privatized Tracer::provider() which tracing-opentelemetry needs to build its PreSampledTracer. Pending a long-term solution for #1571, add a more restricted #[doc(hidden)] Tracer::id_generator() method that only exposes the internal provider's config().id_generator.

This is blocking the 0.24 release.

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

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 74.9%. Comparing base (cdf5f03) to head (428fa81).

Files	Patch %	Lines
opentelemetry-sdk/src/trace/tracer.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1934     +/-   ##
=======================================
- Coverage   74.9%   74.9%   -0.1%     
=======================================
  Files        122     122             
  Lines      20308   20311      +3     
=======================================
  Hits       15217   15217             
- Misses      5091    5094      +3     

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[cijothomas]

Merging quickly to unblock release progress.