fix: Remove pub fields and replace with getter method consistently across …

[cijothomas]

We previously hid the implementation details (eg: Vec!), and exposed getter returning slices. To ensure consistency, this PR changes all fields to be accessible via getter methods only.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 150 lines in your changes missing coverage. Please review.

Project coverage is 81.1%. Comparing base (4be1a32) to head (45708ab).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/data/mod.rs	0.0%	96 Missing ⚠️
opentelemetry-proto/src/transform/metrics.rs	0.0%	32 Missing ⚠️
opentelemetry-stdout/src/metrics/exporter.rs	0.0%	22 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2997     +/-   ##
=======================================
- Coverage   81.5%   81.1%   -0.4%     
=======================================
  Files        126     126             
  Lines      24698   24794     +96     
=======================================
- Hits       20131   20130      -1     
- Misses      4567    4664     +97     

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