Add a saturation metric for redis memory usage compared to the configured maxmemory

As discussed during the team demo, we have maxmemory configured for all of our Redis instances except for redis-persistent/SharedState and redis-sidekiq.

But for most Redis instances except for redis-cache, this is a precaution, we don't really want to remove any keys before their natural expiry. This could mean sessions ending sooner than they should, or rate-limiting not working as expected.

For those Redis instances we should add a saturation metric keeping an eye on how close we are to saturating up to maxmemory, so we make sure that there is enough headroom for natural growth.