Add Prometheus docs

Replaces https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1237
Omnibus MR: https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1227
Omnibus issue: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1481

doc/administration/monitoring/performance/prometheus.md

+# GitLab Prometheus
+
+>**Notes:**
+- Prometheus and the node exporter are bundled in the Omnibus GitLab package
+  since GitLab 8.16. For installations from source you will have to install
+  them yourself. Over subsequent releases additional GitLab metrics will be
+  captured.
+- Prometheus services are off by default but will be on starting with GitLab 9.0.
+
+[Prometheus] is a powerful time-series monitoring service, providing a flexible
+platform for monitoring GitLab and other software products.
+GitLab provides out of the box monitoring with Prometheus, providing easy
+access to high quality time-series monitoring of GitLab services.
+
+## Overview
+
+Prometheus works by periodically connecting to data sources and collecting their
+performance metrics. To view and work with the monitoring data, you can either
+connect directly to Prometheus or utilize a dashboard tool like [Grafana].
+
+## Configuring Prometheus
+
+>**Note:**
+Available since Omnibus GitLab 8.16. For installations from source you'll
+have to install and configure it yourself.
+
+To enable Prometheus:
+
+1. Edit `/etc/gitlab/gitlab.rb`
+1. Find and uncomment the following line, making sure it's set to `true`:
+
+    ```ruby
+    node_exporter['enable'] = true
+    ```
+
+1. Save the file and [reconfigure GitLab][reconfigure] for the changes to
+   take effect
+
+By default, Prometheus will run as the `gitlab-prometheus` user and listen on
+TCP port `9090` under localhost. If the [node exporter](#node-exporter) service
+has been enabled, it will automatically be set up as a monitoring target for
+Prometheus.
+
+## Viewing Performance Metrics
+
+The performance data collected by Prometheus can be viewed directly in the
+Prometheus console or through a compatible dashboard tool.
+The Prometheus interface provides a [flexible query language][prom-query] to work
+with the collected data where you can visualize their output.
+For a more fully featured dashboard, Grafana can be used and has
+[official support for Prometheus][prom-grafana].
+
+## Prometheus exporters
+
+There are a number of libraries and servers which help in exporting existing
+metrics from third-party systems as Prometheus metrics. This is useful for cases
+where it is not feasible to instrument a given system with Prometheus metrics
+directly (for example, HAProxy or Linux system stats). You can read more in the
+[Prometheus exporters and integrations documentation][prom-exporters].
+
+While you can use any exporter you like with your GitLab installation, the
+following ones documented here are bundled in the Omnibus GitLab packages
+making it easy to configure and use.
+
+### Node exporter
+
+>**Note:**
+Available since Omnibus GitLab 8.16. For installations from source you'll
+have to install and configure it yourself.
+
+The [node exporter] allows you to measure various machine resources such as
+memory, disk and CPU utilization.
+
+To enable the node exporter:
+
+1. [Enable Prometheus](#configuring-prometheus)
+1. Edit `/etc/gitlab/gitlab.rb`
+1. Find and uncomment the following line, making sure it's set to `true`:
+
+    ```ruby
+    node_exporter['enable'] = true
+    ```
+
+1. Save the file and [reconfigure GitLab][reconfigure] for the changes to
+   take effect
+
+Prometheus it will now automatically begin collecting performance data from
+the node exporter.
+
+[grafana]: https://grafana.net
+[node exporter]: https://github.com/prometheus/node_exporter
+[prometheus]: https://prometheus.io
+[prom-query]: https://prometheus.io/docs/querying/basics
+[prom-grafana]: https://prometheus.io/docs/visualization/grafana/
+[scrape-config]: https://prometheus.io/docs/operating/configuration/#%3Cscrape_config%3E
+[prom-exporters]: https://prometheus.io/docs/instrumenting/exporters/
+[reconfigure]: ../../restart_gitlab.md#omnibus-gitlab-reconfigure