Add zpool scrub, resilver metrics
Add zpool scrub, resilver metrics

These measure minutes remaining in an active scrub or resilver.
A value of 0 means no active scan. A value of null means the time
remaining is greater than 30 days, so no estimate is provided.

Also add a description of all zpool metrics.

keithf4/nad,hjhart/nad,keithf4/nad,HeinrichHartmann/nad,sean-/nad,circonus-labs/nad,esproul/nad,maier/nad,esproul/nad,maier/nad,maier/nad,sean-/nad,circonus-labs/nad,HeinrichHartmann/nad,hjhart/nad,esproul/nad,sean-/nad,HeinrichHartmann/nad,esproul/nad,esproul/nad,circonus-labs/nad,HeinrichHartmann/nad,keithf4/nad,maier/nad,hjhart/nad,circonus-labs/nad,keithf4/nad,maier/nad,circonus-labs/nad,sean-/nad,HeinrichHartmann/nad,HeinrichHartmann/nad,hjhart/nad,circonus-labs/nad,maier/nad,sean-/nad,keithf4/nad,hjhart/nad