[keyserver] Add metrics for weekly retention

Summary:
We want to introduce new metrics that check retention based on usage in the last week rather than usage in the last day.

Depends on D14040

Test Plan: Ran it on my personal server and confirmed that a message appeared with the new metrics

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D14041