Extend documentation on ServiceMonitor creation (#45071)

The documentation so far only mentions how to disable the ServiceMonitor, leading to the assumption that it is enabled by default.

Getting a service monitor created also requires enabling metrics in keycloak, though:

658faf210f/operator/src/main/java/org/keycloak/operator/controllers/KeycloakServiceMonitorDependentResource.java (L50)

While the missing setting creates a warning, the administrator should already have this information before applying the configuration.

Closes #45070

Signed-off-by: Jens Erat <email@jenserat.de>

docs/guides/operator/advanced-configuration.adoc

@@ -544,6 +544,21 @@ The {project_name} Operator automatically generates a `ServiceMonitor` resource
 `ServiceMonitor` resource to be created, the `monitoring.coreos.com/v1:ServiceMonitor` Custom Resource Definition (CRD)
 must be installed on your {kubernetes} cluster.
 
+A ServiceMonitor is only created if the `metrics-enabled` option is set.
+
+.Enable Keycloak metrics
+[source,yaml]
+----
+apiVersion: k8s.keycloak.org/v2alpha1
+kind: Keycloak
+metadata:
+  name: example-kc
+spec:
+  additionalOptions:
+    - name: metrics-enabled
+      value: "true"
+----
+
 The Operator generates a `ServiceMonitor` with the following spec:
 
 .Default ServiceMonitor