Throughput statistics reporting

We'd like to be able to show some pretty graphs about how much traffic the app is using (I guess maybe both pre- and post-VPN, i.e. log both user traffic and Arti traffic)? @trinity-1686a pointed out the latter can be done easily by just wrapping the TCP sockets.

As for exporting this data; probably just over the JNI, but @ahf also suggested maybe exporting it as prometheus(-style) metrics over HTTP by having the VPN listen on its gateway IP. That might be neat to explore, if only for development and testing purposes.

This should also ideally be per-app, too!