Refer to this documentation for additional configuration options for Prometheus. In this example, I have followed this documentation to implement persistence using the local storage operator]
I followed these steps to configure the local storage operator. Then, I created two LocalVolumes for the two prometheus-k8s StatefulSets via the local storage operator with: oc create -f metrics-volume1.yaml and oc create -f metrics-volume2.yaml, replacing the hostname worker1.yourcluster.domain.com with your own.
Finally, I updated the cluster-monitoring-config ConfigMap with: oc apply -f cluster-monitoring-config.cm.yaml and allowed the prometheus-k8s pods to be recreated.
I verified the PersistentVolumeClaims were created and bound with: oc get pvc -n openshift-monitoring
I also verified the prometheus-k8s pods spun up properly in the project with: oc get pods -o wide -n openshift-monitoring