[keyserver] Introduce metric for weekly active users

Summary:
We already have DAUs and MAUs, but it wouldn't hurt to track this on a weekly basis as well.

Depends on D14042

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

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D14043