fix(async-processor): concurrent exports actually serialised

[alexbrt]

https://github.com/open-telemetry/opentelemetry-rust/pull/2685 unintentionally broke parallel exports by awaiting the export() future directly in opentelemetry-sdk/src/trace/span_processor_with_async_runtime.rs, rather than passing it to the runtime for concurrent polling. As a result, OTEL_BSP_MAX_CONCURRENT_EXPORTS became ineffective, serialising all exports and increasing the risk of dropped spans under load.

This PR restores true parallelism by respecting max_concurrent_exports, and adds tests to verify:

• Exports run in parallel when max_concurrent_exports > 1
• Exports are serialised when max_concurrent_exports == 1

Merge requirement checklist

• [x] CONTRIBUTING guidelines followed
• [x] Unit tests added/updated (if applicable)
• [x] Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• [x] Changes in public API reviewed (if applicable)

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: TommyCpp / name: Zhongyang Wu (8346e30039c291bdf9a1587492fd011e25fd17b2)
• :white_check_mark: login: alexbrt / name: Alexandru-Liviu Bratosin (1905b50d97eef20811e8624d503172834e3f678d, d8e046e7253fb65ff9feb3d13a0e9dd28d8aff75, 2076120fbc16cd4b34eb061f56c94e20e66bcfe3, f7faf804e4572ac47407dfa6c05f340510a77806)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.73684% with 5 lines in your changes missing coverage. Please review.

Project coverage is 80.2%. Comparing base (5e447d0) to head (8346e30). Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...sdk/src/trace/span_processor_with_async_runtime.rs	94.7%	5 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3028     +/-   ##
=======================================
+ Coverage   80.0%   80.2%   +0.1%     
=======================================
  Files        126     126             
  Lines      21879   21949     +70     
=======================================
+ Hits       17519   17604     +85     
+ Misses      4360    4345     -15     

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[alexbrt]

@lalitb @cijothomas Thanks again for all the reviews! 🙏 Is there anything else outstanding that needs to be addressed before this can be merged?

[alexbrt]

Any updates here? :eyes: