doc: add descriptions for helm values.

Add descriptions for the values `helmPreUpgradeCheckerJob` and
`serviceMonitor`.

Ref: 7485

Signed-off-by: James Lu <james.lu@suse.com>

content/docs/1.6.0/monitoring/prometheus-and-grafana-setup.md

@@ -40,6 +40,8 @@ Follow instructions in [Prometheus Operator - Quickstart](https://github.com/pro
 
 ### Install Longhorn ServiceMonitor
 
+#### Install Longhorn ServiceMonitor with Kubectl
+
 1. Create a ServiceMonitor for Longhorn manager.
 
     ```yaml
@@ -61,9 +63,26 @@ Follow instructions in [Prometheus Operator - Quickstart](https://github.com/pro
       - port: manager
     ```
 
-    Longhorn ServiceMonitor has a label selector `app: longhorn-manager` for selecting Longhorn backend service.
+#### Install Longhorn ServiceMonitor with Helm
+
+1. Modify the YAML file `longhorn/chart/values.yaml`.
+
+    ```yaml
+    metrics:
+      serviceMonitor:
+        # -- Setting that allows the creation of a Prometheus ServiceMonitor resource for Longhorn Manager components.
+        enabled: true
+    ```
+
+1. Create a ServiceMonitor for Longhorn manager with Helm
+
+    ```bash
+      helm upgrade longhorn longhorn/longhorn --namespace longhorn-system -f values.yaml
+    ```
 
-    Longhorn ServiceMonitor is included in the Prometheus custom resource so that the Prometheus server can discover all Longhorn manager pods and their endpoints.
+Longhorn ServiceMonitor has a label selector `app: longhorn-manager` for selecting Longhorn backend service.
+
+Longhorn ServiceMonitor is included in the Prometheus custom resource so that the Prometheus server can discover all Longhorn manager pods and their endpoints.
 
 ### Install and configure Prometheus AlertManager
 
@@ -398,4 +417,3 @@ See [Prometheus - Configuration](https://prometheus.io/docs/alerting/latest/conf
 
     You should see the following dashboard at successful setup:
     ![images](/img/screenshots/monitoring/longhorn-example-grafana-dashboard.png)
-

content/docs/1.6.0/references/helm-values.md

@@ -182,8 +182,8 @@ Please also refer to this document [ocp-readme](https://github.com/longhorn/long
 | annotations | `{}` | Annotations to add to the Longhorn Manager DaemonSet Pods. Optional. |
 | enableGoCoverDir | `false` | Enable this to allow Longhorn to generate code coverage profiles |
 | enablePSP | `false` | For Kubernetes < v1.25, if your cluster enables Pod Security Policy admission controller, set this to `true` to ship longhorn-psp which allow privileged Longhorn pods to start |
-| helmPreUpgradeCheckerJob.enabled | `true` |  |
-| metrics.serviceMonitor.enabled | `false` |  |
+| helmPreUpgradeCheckerJob.enabled | `true` | Setting that allows Longhorn to perform pre-upgrade checks. Disable this setting when installing Longhorn using Argo CD or other GitOps solutions. |
+| metrics.serviceMonitor.enabled | `false` | Setting that allows the creation of a Prometheus ServiceMonitor resource for Longhorn Manager components. |
 
 ### System Default Settings