[CONFIGURATION] File configuration - metric aggregation model

[marcalff]

Contributes to #2481

This is a partial fix, that implements supporting declarations.

Changes

Please provide a brief description of the changes here.

• Implement the metric aggregation model for file configuration.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.96%. Comparing base (0bbbdb1) to head (31b827f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3502   +/-   ##
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  Coverage   89.96%   89.96%           
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  Files         219      219           
  Lines        7051     7051           
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  Hits         6343     6343           
  Misses        708      708           

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[dbarker]

LGTM