Document how metrics can be scraped with Prometheus

[draghuram]

As can be seen here: https://catalogicsoftware.com/clab-docs/kubedr/userguide/monitoring.html#prometheus-metrics

We now export some backup related Prometheus metrics. The metrics end point is protected by RBAC by default so I tested it in the following way:

• Removed RBAC by commenting out the line - manager_auth_proxy_patch.yaml and uncommenting the line - manager_prometheus_metrics_patch.yaml in the file kubedr/config/default/kustomization.yaml . You need to build KubeDR after this change.

• Deploy KubeDR and then run the following command:

$ kubectl -n kubedr-system port-forward <KUBEDR-CONTROLLER-POD> 8080:8080

Now, the following command will show all the relevant metrics:

$ curl -s http://localhost:8080/metrics | grep kubedr_

We need to figure out how to set up Prometheus in the default KubeDR configuration. The "kubebuilder" book at https://book.kubebuilder.io/reference/metrics.html has some details in this regard. Once we find a working Prometheus set up, we should document the procedure in user guide. It would also help to have some information on Grafana integration.