chore: OTLP retry post-review cleanup

[scottgerring]

Fixes #
Follow on from #3126

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 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.7%. Comparing base (5c8dbc8) to head (b0475dc).

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/retry.rs	62.5%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3204     +/-   ##
=======================================
- Coverage   80.8%   80.7%   -0.1%     
=======================================
  Files        128     128             
  Lines      23156   23160      +4     
=======================================
+ Hits       18711   18712      +1     
- Misses      4445    4448      +3     

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[scottgerring]

Open for suggestions for where we can put the runtime selection. This strikes me as a broader question about the stabilisation of the runtime mechanism.

https://github.com/scottgerring/opentelemetry-rust/blob/9ba3a06df952d4888009764bebe665281f9e45b7/opentelemetry-otlp/src/exporter/http/mod.rs#L402-L409

[scottgerring]

@cijothomas the build failure split out here has nothing to do with this change:
#3216