Add ServiceMonitor/PodMonitor objects for monitoring

Summary

The Prometheus Operator makes use of

• PodMonitors - https://github.com/prometheus-operator/prometheus-operator/blob/master/Documentation/api.md#podmonitor
• ServiceMonitors - https://github.com/prometheus-operator/prometheus-operator/blob/master/Documentation/api.md#servicemonitor

For adding monitoring to existing Prometheus Operator installations. Consider adding the appropriate items into the GitLab Helm chart such that users of our chart do not need to install their own object definitions for monitoring the GitLab installation.

Current behavior

Installation of the Prometheus Operators requires additional configuration to monitor the GitLab installation.

Expected behavior

If an implemented monitoring option is enabled, the Prometheus Operator will automatically start scraping metrics based on the Service or Pod Monitor objects that are added by our own Helm chart.