Add metrics collection and endpoint

[alonkashtan]

I added metrics endpoint that give two things: (1) Number of packets past so far per proxy (2) Last events happened in Toxiproxy (Client connected, packet sent, packet send failure, Client disconnected).

These endpoints are meant to enable the user to:

1. When running automated requests, verify behaviour of the system (for example, verify that the config doesn't bypass Toxiproxy, that packets were indeed sent, when disconnections occurs and so on)
2. Visualize real-time traffic passing through Toxiproxy. I have been using the fork with a client that shows traffic on a timeline, and this helped us to find performance problems in running automated tests, allowing to cut several minutes off the CD pipeline. Other scenarios were to see in real-time how different actions correlate to chatiness with the DB and finding long forgotten periodic requests