State exposed (tag names):
* name: the interface name ("eth0" etc)
* index: the interface index, "ifindex"
* state: operational state (down, up etc)
* mac: the Ethernet hardware address
* ipv4: the *last* IPv4 address added to the interface
* ipv6: the *last* IPv6 address added to the interface
State we monitor (for changes):
* state
* ipv4 addresses
* ipv6 addresses
