Simplified and optimized logic for calculating per-metric subsampling rate for MapData

[ltiao]

Differential Revision: D66366076

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This pull request was exported from Phabricator. Differential Revision: D66366076

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66366076

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66366076

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66366076

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66366076

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.28571% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.43%. Comparing base (4127865) to head (b7b8491).

Files with missing lines	Patch %	Lines
ax/core/observation.py	87.50%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3106 +/- ## ========================================== + Coverage 95.42% 95.43% +0.01% ========================================== Files 503 503 Lines 50264 50255 -9 ========================================== - Hits 47965 47962 -3 + Misses 2299 2293 -6 ```

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