[Compute usage quotas] (Part 2) Talk to Prometheus and aggregate metrics

[jnywong]

Context

This is a follow up issue to #7547. Here are some follow up points to address from that issue:

• If a user refreshes hub/spawn page after a server launch deny, then this will spam the server logs with 403 errors. How to handle this?
• When the exception is raised in the pre_spawn_hook, jupyterhub.user emits a log message and juptyerhub.handlers.pages emits a log message. This suggests to me there may be a more elegant way to handle this.
• If the user is bumped into a different group to increase quota limits, then we need to refresh their auth state before spawn
  • validate refresh_pre_spawn works as expected

Definition of Done

• Convert library to a service
• Write an exporter to expose usage and quota metrics (to be consumed by JH service)
• Code review