Include Redis Exporter for Prometheus
In order to more easily export metrics on the health and responsiveness of GitLab services, we should include the Redis exporter for Prometheus. This will provide metrics on the health of the Redis service which is packaged.
This exporter should be turned off by default for right now, with a note indicating we will make it on by default for 9.0.
This issue is a component of the larger metrics gathering effort in #1862 (closed).