fix: Avoid exposing HistogramBuckets and bounds

[cijothomas]

Last part of #2968

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.

Project coverage is 81.3%. Comparing base (fed6fee) to head (2390c86).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-stdout/src/metrics/exporter.rs	0.0%	13 Missing ⚠️
opentelemetry-sdk/src/metrics/data/mod.rs	0.0%	6 Missing ⚠️
opentelemetry-proto/src/transform/metrics.rs	0.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2969     +/-   ##
=======================================
- Coverage   81.3%   81.3%   -0.1%     
=======================================
  Files        126     126             
  Lines      24356   24364      +8     
=======================================
  Hits       19816   19816             
- Misses      4540    4548      +8     

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