Use apply_to_collection in metrics_to_scalars

[carmocca]

What does this PR do?

Part of #7882

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[codecov]

Codecov Report

Merging #7888 (5456561) into master (4f3af42) will decrease coverage by 0%.
The diff coverage is 100%.

❗ Current head 5456561 differs from pull request most recent head 784668b. Consider uploading reports for the commit 784668b to get more accurate results

@@          Coverage Diff           @@
##           master   #7888   +/-   ##
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- Coverage      88%     88%   -0%     
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  Files         202     202           
  Lines       13161   13159    -2     
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- Hits        11566   11526   -40     
- Misses       1595    1633   +38     

[ethanwharris]

LGTM 😃

[tchaton]

LGMT !