[inputs.nftables] Monitor set element counts

[niklasf]

Use Case

Similar to inputs.ipset for iptables, in nftables named sets can be dynamically modified, for example to ban ip addresses. This is useful to monitor.

Expected behavior

For a set like

$ sudo nft list table inet filter

table inet filter {
	set named_set {
		type ipv6_addr
		size 65536	# count 3
		timeout 5m
		elements = { ::3 expires 4m47s406ms,
			         ::4 expires 4m44s399ms,
			         ::5 expires 4m42s65ms }
	}
    # ...
}

the nftables.input plugin could provide:

nftables,set=named_set,table=filter count=3i 1763040447356078375

Actual behavior

Not yet implemented.

Additional info

No response