To allow Check_MK to inventorize and fetch data for the zfs_arc_cache plugin, just create a file like /usr/lib/check_mk_agent/plugins/zfs_arc_cache with the content from below and make it executable.

#!/bin/sh
echo "<<<zfs_arc_cache>>>"
cat /proc/spl/kstat/zfs/arcstats | tail -n +3 | awk '{print $1" = "$NF}'

You then should get some nice graphs showing all the stats.

ZFS ARC cache in Check_MK