Merge branch '327670-define-possible-redis-slots-for-events-tracked-using-redis-hll' into 'master'

Proposal for Redis slots name for events tracked using Redis HLL

See merge request gitlab-org/gitlab!60477

doc/development/usage_ping/index.md

@@ -635,11 +635,12 @@ Implemented using Redis methods [PFADD](https://redis.io/commands/pfadd) and [PF
 
    - `category`: event category. Used for getting total counts for events in a category, for easier
      access to a group of events.
-   - `redis_slot`: optional Redis slot; default value: event name. Used if needed to calculate totals
-     for a group of metrics. Ensure keys are in the same slot. For example:
+   - `redis_slot`: optional Redis slot. Default value: event name. Only event data that is stored in the same slot
+     can be aggregated. Ensure keys are in the same slot. For example:
      `users_creating_epics` with `redis_slot: 'users'` builds Redis key
      `{users}_creating_epics-2020-34`. If `redis_slot` is not defined the Redis key will
      be `{users_creating_epics}-2020-34`.
+     Recommended slots to use are: `users`, `projects`. This is the value we count.
    - `expiry`: expiry time in days. Default: 29 days for daily aggregation and 6 weeks for weekly
      aggregation.
    - `aggregation`: may be set to a `:daily` or `:weekly` key. Defines how counting data is stored in Redis.