/api/metrics for producing various statistics

We could have an API endpoint that produces some statistics of REMS usage.
It could be like `/api/health` but it could compute more expensive statistics, like:

- number of applications
- number of application events
- number of events in an application
- number of applications per applicant
- average time an application spends in various states, like draft, submitted, etc.
- and more!

NB: require an API key for this information, like reporter?