feat(proxy): add failed connection count stats

For the summary log and for Prometheus metrics.

Log output example:

> In the last 1h0m0s, there were 7 completed successful connections. 2 connections failed. Traffic Relayed ↓ 321 KB (0.10 KB/s), ↑ 123 KB (0.05 KB/s).

proxy/lib/snowflake.go

@@ -682,6 +682,9 @@ func (sf *SnowflakeProxy) runSession(sid string) {
 		connectedToClient = true
 	case <-time.After(dataChannelTimeout):
 		log.Println("Timed out waiting for client to open data channel.")
+		sf.EventDispatcher.OnNewSnowflakeEvent(
+			event.EventOnProxyConnectionFailed{},
+		)
 		if err := pc.Close(); err != nil {
 			log.Printf("error calling pc.Close: %v", err)
 		}