metrics: add networkd related metrics (#40619)

This adds support for networkd related metrics. The output looks like this:
```
{
	"name" : "io.systemd.Network.addressState",
	"object" : "lo",
	"value" : "off"
}
{
	"name" : "io.systemd.Network.adminState",
	"object" : "lo",
	"value" : "unmanaged"
}
{
	"name" : "io.systemd.Network.carrierState",
	"object" : "lo",
	"value" : "carrier"
}
{
	"name" : "io.systemd.Network.ipv4AddressState",
	"object" : "lo",
	"value" : "off"
}
{
	"name" : "io.systemd.Network.ipv6AddressState",
	"object" : "lo",
	"value" : "off"
}
{
	"name" : "io.systemd.Network.managedInterfaces",
	"value" : 0
}
{
	"name" : "io.systemd.Network.operationalState",
	"object" : "lo",
	"value" : "carrier"
}
```