This exposes some of the info from /api/status in the prometheus
metrics endpoint, so they can be scraped over time and queried.

- Server started timestamp, so we can easily see how long servers
  have been running
- Last activity timestamp, so we can see when users were actually
  active
- User activity duration (in seconds), so we can more easily build
  reports of how long users are actually active over time.

All these would be very helpful in JupyterHub environments.

Follow-up to jupyter-server#1470

for more information, see https://pre-commit.ci