Expose disk used and available stats to prometheus via sigar

For MB-57370 we are exposing the disk size and usage to prometheus from erlang's disksup. This uses the same prefix as our sigar disk stats, and might be better fetched with sigar instead of disksup as it would ensure that we have all our disk stats coming from the same place and allow us to get a greater set of disk stats (currently we can only get size and usage, and the available space can determined using these values).