Documentation for Prometheus Service Discovery

Closes #63110

See merge request gitlab-org/gitlab-ce!29811

(cherry picked from commit 46640168)

95525a56 Adding documentation for Prometheus Service Discovery
781d05af Moved monitoring to its own step
f6c0fd2e Adding monitoring to number list in Redis
db2f83f3 Updates based on review
1105b8ea A couple of missed adjustments