[mongo] add comments to metrics to indicate which collector it comes from

[lu-zhengda]

What does this PR do?

This PR adds comments to metrics to indicate which collector it comes from

Motivation

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Changelog entries must be created for modifications to shipped code
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.58%. Comparing base (d5250d3) to head (813b3eb).
Report is 3 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
hive	?
hivemq	?
ignite	?
jboss_wildfly	?
kafka	?
mongo	93.49% <ø> (-0.03%)	⬇️
presto	?
solr	?

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