Redis exporter

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

The Redis exporter allows you to measure various Redis metrics. For more information on what's exported, read the upstream documentation.

To enable the Redis exporter:

Enable Prometheus
Edit /etc/gitlab/gitlab.rb
Add or find and uncomment the following line, making sure it's set to true:
```
redis_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 Redis exporter exposed under localhost:9121.