## About the changes
- `getActiveUsers` is using multiple stores, so it is refactored into
read-model
- Refactored Instance stats service into `features` to co-locate related
code
Closes https://linear.app/unleash/issue/UNL-230/active-users-prometheus
### Important files
`src/lib/features/instance-stats/getActiveUsers.ts`
## Discussion points
`getActiveUsers` is coded less _class-based_ then previous similar
read-models. In one file instead of 3 (read-model interface, fake read
model, sql read model). I find types and functions way more readable,
but I'm ready to refactor it to interfaces and classes if consistency is
more important.
## 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
```
