Add docs on PgBouncer exporter

doc/administration/monitoring/prometheus/index.md

@@ -157,6 +157,12 @@ The Postgres exporter allows you to measure various PostgreSQL metrics.
 
 [➔ Read more about the Postgres exporter.](postgres_exporter.md)
 
+### PgBouncer exporter
+
+The PgBouncer exporter allows you to measure various PgBouncer metrics.
+
+[➔ Read more about the PgBouncer exporter.](pgbouncer_exporter.md)
+
 ### GitLab monitor exporter
 
 The GitLab monitor exporter allows you to measure various GitLab metrics, pulled from Redis and the database.

doc/administration/monitoring/prometheus/pgbouncer_exporter.md

+# PgBouncer exporter
+
+>**Note:**
+Available since [Omnibus GitLab 11.0][2493]. For installations from source
+you'll have to install and configure it yourself.
+
+The [PgBouncer exporter] allows you to measure various PgBouncer metrics.
+
+To enable the PgBouncer exporter:
+
+1. [Enable Prometheus](index.md#configuring-prometheus)
+1. Edit `/etc/gitlab/gitlab.rb`
+1. Add or find and uncomment the following line, making sure it's set to `true`:
+
+    ```ruby
+    pgbouncer_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 PgBouncer exporter exposed under `localhost:9188`.
+
+The PgBouncer exporter will also be enabled by default if the [pgbouncer_role][postgres roles]
+is enabled.
+
+[← Back to the main Prometheus page](index.md)
+
+[2493]: https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/2493
+[PgBouncer exporter]: https://github.com/stanhu/pgbouncer_exporter
+[postgres roles]: https://docs.gitlab.com/omnibus/roles/#postgres-roles
+[prometheus]: https://prometheus.io
+[reconfigure]: ../../restart_gitlab.md#omnibus-gitlab-reconfigure