Exclude snapshots from ZFS filesystems in the discovery

Currently, if snapdir is visible, then zfs snapshots are discovered and items/triggers/graphs are uselessly created for them. Snapshots could completely be excluded from discovery, all these metrics are irrelevant on them.

Apart from that, in same rare cases this can result in some 4-6K additional items on a system, needlessly increasing the vps on the server - these metrics are not doing anything, except for clogging up server resources.