Update documentation on how to monitor node-agent and maintenance pods

[lanak01]

Add documentation https://github.com/openshift/oadp-operator/blob/master/docs/oadp_monitoring.md on how to enable workload monitoring for node-agent pods:

Add labels to nodeAgent.podConfig

configuration:
    nodeAgent:
      enable: true
      uploaderType: "kopia"
      podConfig:
        labels:
          app.kubernetes.io/component: server
          app.kubernetes.io/instance: velero
          app.kubernetes.io/managed-by: oadp-operator
          app.kubernetes.io/name: velero
          openshift.io/oadp: 'True'

The labels must match the labels in Service that provides network endpoint for nodeAgent pods.
OADP service openshift-adp-velero-metrics-svc:

  selector:
    app.kubernetes.io/component: server
    app.kubernetes.io/instance: velero
    app.kubernetes.io/managed-by: oadp-operator
    app.kubernetes.io/name: velero
    openshift.io/oadp: 'True'

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[openshift-bot]

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