Fix Redis pool hit ratio graph on Grafana

Problem

Currently, we're using the following query for the connection pool hit ratio metric on the Redis Grafana dashboard (here):

sum(max_over_time(registry_redis_pool_stats_hits{environment="gprd", cluster=~".*", stage="main", exported_instance=~".*"}[$__interval]))

We should not use max_over_time but rather rate as we want to display the rate of pool hits.