📖 describe need of RBAC permissions to allow Prometheus Operator to have access to the metrics (#2827)

* Describe the need of RBAC permissions to allow Prometheus Operator to have access to the metrics

Signed-off-by: João Vilaça <[email protected]>

* Apply suggestions from code review

Co-authored-by: Aldo Culquicondor <[email protected]>

Co-authored-by: Aldo Culquicondor <[email protected]>

Author: machadovilaca

docs/book/src/reference/metrics.md

@@ -51,6 +51,7 @@ Follow the steps below to export the metrics using the Prometheus Operator:
    We recommend using [kube-prometheus](https://github.com/coreos/kube-prometheus#installing)
    in production if you don't have your own monitoring system.
    If you are just experimenting, you can only install Prometheus and Prometheus Operator.
+
 2. Uncomment the line `- ../prometheus` in the `config/default/kustomization.yaml`.
    It creates the `ServiceMonitor` resource which enables exporting the metrics.
 
@@ -69,6 +70,20 @@ NAME                                         AGE
 monitor-controller-manager-metrics-monitor   2m8s
 ```
 
+<aside class="warning">
+<h2>If you are using Prometheus Operator ensure that you have the required
+permissions</h2>
+
+If you are using Prometheus Operator, be aware that, by default, its RBAC
+rules are only enabled for the `default` and `kube-system namespaces`. See its
+guide to know [how to configure kube-prometheus to monitor other namespaces using the `.jsonnet` file](https://github.com/prometheus-operator/kube-prometheus/blob/main/docs/monitoring-other-namespaces.md).
+
+Alternatively, you can give the Prometheus Operator permissions to monitor other namespaces using RBAC. See the Prometheus Operator
+[Enable RBAC rules for Prometheus pods](https://github.com/prometheus-operator/prometheus-operator/blob/main/Documentation/user-guides/getting-started.md#enable-rbac-rules-for-prometheus-pods)
+documentation to know how to enable the permissions on the namespace where the
+`ServiceMonitor` and manager exist.
+</aside>
+
 Also, notice that the metrics are exported by default through port `8443`. In this way,
 you are able to check the Prometheus metrics in its dashboard. To verify it, search
 for the metrics exported from the namespace where the project is running