derp: add debug traffic handler

This adds a handler on the DERP server for logging bytes send and received by clients of the
server, by holding open a connection and recording if there is a difference between the number
of bytes sent and received. It sends a JSON marshalled object if there is an increase in the
number of bytes.

Signed-off-by: julianknodt <julianknodt@gmail.com>

cmd/derper/derper.go

@@ -171,6 +171,7 @@ func main() {
 			io.WriteString(w, "derp.Server ConsistencyCheck okay")
 		}
 	}))
+	debug.Handle("traffic", "Traffic check", http.HandlerFunc(s.ServeDebugTraffic))
 
 	if *runSTUN {
 		go serveSTUN()