xnet: Add event corresponding to network dial start

We already have TraceConnect which corresponds to event of established
network connection.

However in many test scenarios it is required to know and pause
execution right before dial is going to happen. For this introduce
TraceDial which represents event corresponding to network dialing start.

xnet/trace.go

@@ -47,12 +47,19 @@ func NetTrace(inner Networker, tracer Tracer) Networker {
 
 // Tracer is the interface that needs to be implemented by network trace receivers
 type Tracer interface {
+	TraceNetDial(*TraceDial)
 	TraceNetConnect(*TraceConnect)
 	TraceNetListen(*TraceListen)
 	TraceNetTx(*TraceTx)
 }
 
-// TraceConnect is event corresponding to network connection
+// TraceDial is event corresponding to network dial start.
+type TraceDial struct {
+	// XXX also put networker?
+	Dialer, Addr string
+}
+
+// TraceConnect is event corresponding to established network connection.
 type TraceConnect struct {
 	// XXX also put networker?
 	Src, Dst net.Addr
@@ -88,7 +95,7 @@ func (nt *netTrace) Name() string {
 }
 
 func (nt *netTrace) Dial(ctx context.Context, addr string) (net.Conn, error) {
-	// XXX +TraceNetDialPost ?
+	nt.tracer.TraceNetDial(&TraceDial{Dialer: nt.inner.Name(), Addr: addr})
 	c, err := nt.inner.Dial(ctx, addr)
 	if err != nil {
 		return nil, err