Add Prometheus scrape config for `cable` service
With gitlab!41358 (merged) we will have Prometheus metrics for ActionCable.
This change adds the new endpoint as a target.
Note that this is only relevant when AC runs in standalone mode i.e. on a dedicated server; for in-app mode, metrics will be rolled into those exported by the Puma instance it is running in.
It is safe to merge this MR prior to landing gitlab!41358 (merged); that target will just always be reported as down.