OtlpTracePipeline fixed to no longer set global provider

[utpilla]

Towards #1592

Changes

• This PR modifies the OTLP trace pipeline to not setup the global meter provider itself.
• End users must use the returned provider (a clone of it, if they cannot give up ownership), and call global::set_tracer_provider() themselves, if they want to set a global meter provider.
• Updated documentation comments for logs

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)

[TommyCpp]

Good to merge once we rename all init_tracer to init_tracer_provider. Thanks a lot!

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 73.8%. Comparing base (ec9fd62) to head (9283384).
Report is 1 commits behind head on main.

Files	Patch %	Lines
opentelemetry-otlp/src/span.rs	50.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1812     +/-   ##
=======================================
+ Coverage   73.6%   73.8%   +0.1%     
=======================================
  Files        122     122             
  Lines      19515   19503     -12     
=======================================
+ Hits       14377   14402     +25     
+ Misses      5138    5101     -37     

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[utpilla]

Thanks for the review! I have addressed all the comments.