Move rails cache SLI from redis-cache to redis-cluster-cache
Now that redis-cluster-cache is fully operational, we should move rails cache SLI over. Defining it in both services is misleading since the metric is emitted regardless of feature-flag state since it is at the Rails.cache layer (no insight into Redis client's state).
i.e. if Rails.cache was using a Multistore with dual-write enabled, the subscriber increases the count by 1. If the multistore's default store is toggled, the same metric is emitted. This is of a lower concern since we have rails-redis-client SLI which is a super-set of rails cache's metrics.