feat(otlp): add build() on base exporter builders to auto-select from env

[scottgerring]

Fixes #3208.
With this change, a user can simply call build() on an exporter instead of having to explicitly choose tonic or http. Because we are only implementing this for NoExporterBuilderSet this doesn't cause ambiguity with the existing building pattern.

Changes

Please provide a brief description of the changes here.

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

❌ Patch coverage is 80.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.1%. Comparing base (d10b025) to head (0341d48).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/logs.rs	80.0%	2 Missing ⚠️
opentelemetry-otlp/src/metric.rs	80.0%	2 Missing ⚠️
opentelemetry-otlp/src/span.rs	80.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3394     +/-   ##
=======================================
+ Coverage   82.7%   83.1%   +0.4%     
=======================================
  Files        128     128             
  Lines      24811   24841     +30     
=======================================
+ Hits       20526   20652    +126     
+ Misses      4285    4189     -96     

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[cijothomas]

Lets add a changelog, and see if we can leverage this in the otlp examples in the repo?

[lalitb]

Thanks, LGTM once changelog and tests are added.

A nice follow-up could be to support something like .with_endpoint("...").build() directly on the base builder, without having to pick with_tonic() or with_http() first. That way users can set things like endpoint or timeout in code while still letting the transport get picked automatically.

[scottgerring]

@cijothomas @lalitb feedback addressed should be good to go