Do not alert on split brain for the ZFS patroni node type

As seen in gitlab-com/gl-infra/production#4442 (closed), the ZFS patroni node is chain replicated off one of the other replicas and consequently the split brain alert fires because there are two visible upstream DB hosts in the visible query, even though this is fine in this case

By excluding type=zfs, the alert expression will focus on the core DB replicas which are expceted to be replicating only off the primary DB