Add GitLab monitor exporter docs

doc/administration/monitoring/prometheus/gitlab_monitor_exporter.md

+# GitLab monitor exporter
+
+>**Note:**
+Available since [Omnibus GitLab 8.17][1132]. For installations from source
+you'll have to install and configure it yourself.
+
+The [GitLab monitor exporter] allows you to measure various GitLab metrics.
+
+To enable the GitLab monitor exporter:
+
+1. [Enable Prometheus](index.md#configuring-prometheus)
+1. Edit `/etc/gitlab/gitlab.rb`
+1. Find and uncomment the following line, making sure it's set to `true`:
+
+    ```ruby
+    gitlab_exporter['enable'] = true
+    ```
+
+1. Save the file and [reconfigure GitLab][reconfigure] for the changes to
+   take effect
+
+Prometheus will now automatically begin collecting performance data from
+the GitLab monitor exporter exposed under `localhost:9168`.
+
+[← Back to the main Prometheus page](index.md)
+
+[1132]: https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1132
+[GitLab monitor exporter]: https://gitlab.com/gitlab-org/gitlab-monitor
+[prometheus]: https://prometheus.io
+[reconfigure]: ../../restart_gitlab.md#omnibus-gitlab-reconfigure

doc/administration/monitoring/prometheus/index.md

@@ -87,6 +87,12 @@ The Postgres exporter allows you to measure various PostgreSQL metrics.
 
 [➔ Read more about the Postgres exporter.](postgres_exporter.md)
 
+### GitLab monitor exporter
+
+The GitLab monitor exporter allows you to measure various GitLab metrics.
+
+[➔ Read more about the GitLab monitor exporter.](gitlab_monitor_exporter.md)
+
 [grafana]: https://grafana.net
 [prometheus]: https://prometheus.io
 [prom-query]: https://prometheus.io/docs/querying/basics