Track Redis latency (apdex)

We currently don't have an apdex tile on our Redis dashboard. https://dashboards.gitlab.net/d/redis-main/redis-overview?orgId=1

I think it would be good to have one.

We can add a Redis latency histogram to our existing request and error counters (added in gitlab-org/gitlab!35992 (merged)), which will have a label per-storage. These should have buckets only for the threshold values: #476

Once we've done that, we can create new issues to:

1. Change the existing Redis cache apdex (#439 (comment 364553093)) to use the new histogram.
2. Add apdex panels for the other Redis instances.