Postgres exporter

Note: Available since Omnibus GitLab 8.17. For installations from source you'll have to install and configure it yourself.

The postgres exporter allows you to measure various PostgreSQL metrics.

To enable the postgres exporter:

Enable Prometheus
Edit /etc/gitlab/gitlab.rb
Add or find and uncomment the following line, making sure it's set to true:
```
postgres_exporter['enable'] = true
```
Save the file and reconfigure GitLab for the changes to take effect

Prometheus will now automatically begin collecting performance data from the postgres exporter exposed under localhost:9187.