## About the changes
Introduce a snapshot version of instanceStats inside
instance-stats-service to provide a cached state of the statistics
without compromising the DB.
### Important notes
Some rule-of-thumb applied in the PR that can be changed:
1. The snapshot refresh time
2. The threshold to report appName with the metrics
## Discussion points
1. The snapshot could be limited to just the information needed (things
like `hasOIDC` don't change until there's a restart), to optimize the memory usage
3. metrics.ts (used to expose Prometheus metrics) has a [refresh
interval of
2hs](2d16730cc2/src/lib/metrics.ts (L189-L195)),
but with this implementation, we could remove that background task and
rely on the snapshot
4. We could additionally update the snapshot every time someone queries
the DB to fetch stats (`getStats()` method), but it may increase
complexity without a significant benefit
Co-authored-by: Mateusz Kwasniewski <kwasniewski.mateusz@gmail.com>
Co-authored-by: Simon Hornby <liquidwicked64@gmail.com>
## About the changes
This metric will expose an aggregated view of how many client
applications are registered in Unleash. Since applications are ephemeral
we are exposing this metric in different time windows based on when the
application was last seen.
The caveat is that we issue a database query for each new range we want
to add. Hopefully, this should not be a problem because:
a) the amount of ranges we'd expose is small and unlikely to grow
b) this is currently updated at startup time and even if we update it on
a scheduled basis the refresh rate will be rather sparse
## Sample data
This is how metrics will look like
```
# HELP client_apps_total Number of registered client apps aggregated by range by last seen
# TYPE client_apps_total gauge
client_apps_total{range="allTime"} 3
client_apps_total{range="30d"} 3
client_apps_total{range="7d"} 2
```
