refactor: prioritize code-based config over env vars in OTLP exporter

[gruebel]

Fixes #2818

Changes

• had to make the timeout field optional to properly handle code-based config prefer over env vars

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 97.20670% with 5 lines in your changes missing coverage. Please review.

Project coverage is 80.4%. Comparing base (06abe3d) to head (5f101d8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/mod.rs	96.1%	2 Missing ⚠️
opentelemetry-otlp/src/exporter/tonic/mod.rs	97.6%	2 Missing ⚠️
opentelemetry-otlp/src/exporter/http/mod.rs	97.6%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2827     +/-   ##
=======================================
+ Coverage   79.7%   80.4%   +0.7%     
=======================================
  Files        124     124             
  Lines      23297   23390     +93     
=======================================
+ Hits       18578   18828    +250     
+ Misses      4719    4562    -157     

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[cijothomas]

LGTM. Please modify the docs and changelog with more details so users will have easier time navigating this change.

[cijothomas]

@TommyCpp Could you also double check? I recall you suggested to add a with_env_preference like API to make it easier to migrate, but it looks like we preferred ENV over code only for OTLP, and that too for 2-3 settings, which make me think there is no need of additional convenience method to revert to old behavior.. Please take a look and see if this is okay!

[TommyCpp]

but it looks like we preferred ENV over code only for OTLP, and that too for 2-3 settings, which make me think there is no need of additional convenience method to revert to old behavior

Yep if that's the case then sounds fair to me