Expose XFS inode statistics

We use XFS on ~200 disks in our Openstack Swift cluster, hosting a few billion files.   Most of our RAM on our 10 or so servers is dedicated to the XFS inode cache,  and hit / miss / reclaim / recycle counters are very useful to me.

The counters are already implemented in github.com/prometheus/procfs/xfs;  I've prepared a pull request that just exposes these, and adds test fixtures. 