[WIP] BatchSpanProcessor with dedicated thread.

[lalitb]

Fixes #2454
Design discussion issue (if applicable) #

Changes

WIP, tentative ready by Friday eod. changes consistent with #2436

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 83.45070% with 94 lines in your changes missing coverage. Please review.

Project coverage is 77.0%. Comparing base (6209c06) to head (3feaddc).

Files with missing lines	Patch %	Lines
...sdk/src/trace/span_processor_with_async_runtime.rs	81.5%	60 Missing ⚠️
opentelemetry-sdk/src/trace/span_processor.rs	89.2%	25 Missing ⚠️
opentelemetry-sdk/src/trace/runtime_tests.rs	0.0%	5 Missing ⚠️
opentelemetry-sdk/src/trace/provider.rs	0.0%	2 Missing ⚠️
opentelemetry-zipkin/src/exporter/mod.rs	0.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2456     +/-   ##
=======================================
+ Coverage   76.9%   77.0%   +0.1%     
=======================================
  Files        123     124      +1     
  Lines      22581   22843    +262     
=======================================
+ Hits       17379   17609    +230     
- Misses      5202    5234     +32     

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Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

Using unwrap on the mutex lock can potentially panic. Consider handling this more gracefully to avoid unexpected panics.

Suggested change

	handle.join().expect("Failed to join thread");
	handle.join().expect("Failed to join thread after mutex lock");

[lalitb]

Nice to have these basic issues getting caught :)

[lalitb]

most stuff should work, apart from integration tests. Will be looking into it. Will make this ready for review after that.