This reports the total count of users (split by appservice) which is
meant to be the monthless counterpart to the MAU metric.
Context:
> So this is largely for billing purposes and wanting to know the change
in the number of users. If a user is deactivated then we no longer want
to count them. Consumers *might* want to count appservice users, and
maybe count them based on the service (perhaps you change more for users
under bridge X or bridge Y).
>
> *-- https://github.com/element-hq/synapse/pull/19848#discussion_r3402216234*