[keyserver] Introduce cronjob for product metrics

Summary: This cronjob will generate some product metrics daily and send them to me in a Comm chat.

Test Plan: Ran it via script on the auxiliary production keyserver instance running on my personal server and confirmed that it works

Reviewers: will, varun

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13751