Add catch-all unit test for metric cleanup

[justinjung04]

Is your feature request related to a problem? Please describe.
When we add a new metric, or make adjustment to existing metrics, it could be easy for us to miss them in the cleaning logic, resulting in dead metrics piling up.

Describe the solution you'd like
It will be nice to have a catch-all unit test as a guardrail for human errors.

Describe alternatives you've considered
An alternative is to add a new PR checklist "if it's a new metric, please don't forget to clean it up" but a unit test will be much better.

Additional context
#6698