[keyserver] Introduce metrics for number of message authors

Summary:
We want to track the number of unique message senders as a metric to report externally.

Depends on D14041

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/D14042