print stats on spares and caches

[richardelling]

Currently, zpool_prometheus follows the vdev children, which represent the currently active devices in pools. Auxiliary devices, such as spares and caches, are not shown unless or until the spare is activated in a pool.

Questions:

1. do we care about information on spares that are not active?
2. how do we reconcile duplicate spare devices shared amongst pools?
3. do we care about information on caches?