Add Prometheus docs (!8699) · Merge requests · GitLab.org / GitLab FOSS · GitLab

Merged Achilleas Pipinellis (OOO | back on july 25th) requested to merge docs/prometheus into master Jan 22, 2017

Replaces omnibus-gitlab!1237 (merged)
Omnibus MR: omnibus-gitlab!1227 (merged)
Omnibus issue: omnibus-gitlab#1481 (closed)