Add a reporter to report on the available space in the filesystem path
being used as the rootDir of the dbnode.

This can be used for alerting purposes.

The code change for this is here:
shrek@2d1f72e

Per the contribution guidelines, I am opening this issue for discussion prior to opening a PR.

There is a grafana panel in Node Details dashboard already called "Disk Free Space" which is currently not implemented. That panel could be completed with these metrics.