Issue 1754: improve the documentation about OADP Monitoring (#1764)

* Improve the documentation

* Update docs/oadp_monitoring.md

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Co-authored-by: Tiger Kaovilai <[email protected]>

Author: hu-keyu

docs/oadp_monitoring.md

@@ -20,6 +20,8 @@ Monitoring [metrics](#metrics) requires enabling monitoring for the user-defined
 ### Prerequisites
 * OADP operator, a credentials secret, and a DataProtectionApplication (DPA) CR are all created. Follow [these steps](/docs/install_olm.md) for installation instructions.
 * shell with the `oc` CLI command available and admin level access to the OpenShift cluster.
+* You have created a [cluster monitoring config map](https://docs.redhat.com/en/documentation/openshift_container_platform/latest/html/monitoring/configuring-core-platform-monitoring#creating-cluster-monitoring-configmap_preparing-to-configure-the-monitoring-stack).
+
 
 
 ### Enable and Configure User Workload Monitoring