fix: Expose SpanExporterBuilder and MetricExporterBuilder

[elichai]

Fixes #2358

Changes

Exposes SpanExporterBuilder and MetricExporterBuilder
Currently these are hidden even though they are exposed: https://docs.rs/opentelemetry-otlp/latest/opentelemetry_otlp/struct.SpanExporter.html#method.builder

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: elichai / name: Elichai Turkel (664b19e)
• ✅ login: cijothomas / name: Cijo Thomas (59c09b9)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.4%. Comparing base (cc93ead) to head (59c09b9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2966   +/-   ##
=====================================
  Coverage   81.4%   81.4%           
=====================================
  Files        126     126           
  Lines      24571   24571           
=====================================
+ Hits       20001   20005    +4     
+ Misses      4570    4566    -4     

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