This commit updates the Interface.Inside type to be a new interface
type instead of a *Tun. This will allow for an inside interface
that does not use a tun device, such as a single-binary client that
can run without elevated privileges.
This change add more metrics around "meta" (non "message" type packets).
For lighthouse packets, we also record statistics around the specific
lighthouse meta type.
We don't keep statistics for the "message" type so that we don't slow
down the fast path (and you can just look at metrics on the tun
interface to find that information).
