Missing metrics for ephemeral storage in OpenShift

Issue

• Metrics for ephemeral storage (used for instance by volume type emptyDir) are not available in Prometheus. Node exporter 0.16 used in OCP 3.10 and 3.11 does not expose node_filesystem_avail_bytes metrics and in general all file system metrics for the mounting point /var/lib/origin/openshift.local.volumes. Storage exhaustion may cause pod eviction and prevent admission of new pods until sufficient storage has been reclaimed.