reminder (backend) service metrics #21
Description
Right now all reminder subscriptions are stored in a single array, which keeps the code simple but makes most operations linear in the number of active subscriptions (across all users). There is a potential issue with stale subscriptions piling up and slowing the backend or it just being too slow in general, so provide a way to determine if this is indeed an issue.
This could be some endpoint like /metrics/reminders that responds with the number of active subscriptions and a sample of the latencies of recent requests. I can make something that polls it and aggregates the data, or maybe this should be combined with the other data collection work happening.