[OTLP Exporter] Mark a few APIs private

[utpilla]

Changes

• Marked a few APIs private

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 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.1%. Comparing base (ea4b5e4) to head (40ff4f8).

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/logs.rs	0.0%	1 Missing ⚠️
opentelemetry-otlp/src/metric.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2229   +/-   ##
=====================================
  Coverage   79.1%   79.1%           
=====================================
  Files        121     121           
  Lines      21082   21082           
=====================================
  Hits       16680   16680           
  Misses      4402    4402           

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