Added metrics docstrings

[rsyue]

What does this PR do? Please describe: Added docstrings to files in the metrics module

Fixes #1360

Check list:

• [x] Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• [x] Did you read the contributor guideline?
• [x] Did you make sure that your PR does only one thing instead of bundling different changes together?
• [ ] Did you make sure to update the documentation with your changes? (if necessary)
• [ ] Did you write any new necessary tests?
• [x] Did you verify new and existing tests pass locally with your changes?
• [x] Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)