Write an exporter for CephFS usage data

It would be nice to be able to get usage metrics per subdirectory of the CephFS filesystem. The same way we get stats from Kubernetes PVCs.

The xattr ceph.dir.rbytes (and rfiles, rsubdirs, rentries) can give a recursive total without enumerating the fileystem.

cephfs-xattr-influx is an example of such an exporter but with a static list of targets and writing to InfluxDB not Prometheus.