fix: MetricExporters use getter methods instead of direct access

[cijothomas]

Fixes #
Design discussion issue (if applicable) #

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

Attention: Patch coverage is 0% with 46 lines in your changes missing coverage. Please review.

Project coverage is 81.2%. Comparing base (8499914) to head (17cca6e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/data/mod.rs	0.0%	18 Missing ⚠️
opentelemetry-stdout/src/metrics/exporter.rs	0.0%	16 Missing ⚠️
opentelemetry-proto/src/transform/metrics.rs	0.0%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2973     +/-   ##
=======================================
- Coverage   81.3%   81.2%   -0.1%     
=======================================
  Files        126     126             
  Lines      24375   24394     +19     
=======================================
+ Hits       19827   19828      +1     
- Misses      4548    4566     +18     

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[utpilla]

The individual instrument and their datapoint related structs (such as Sum, SumDataPoint, etc.) also have some public fields where we can use getters.

[hdost]

It could be nice to have include rationale in PRs.