refactor: move bytecounting conn in bytecounter pkg (#392)

* refactor: move bytecounting conn in bytecounter pkg

This enables other pieces of code to request bytecounting without
depending on netx or on the perverse using-the-context-to-configure-
byte-counting mechanism.

Also occurred when working on https://github.com/ooni/probe/issues/1687

* fix: add missing docs

internal/engine/netx/dialer/bytecounter.go

@@ -20,12 +20,13 @@ func (d *byteCounterDialer) DialContext(
 	if err != nil {
 		return nil, err
 	}
-	exp := contextExperimentByteCounter(ctx)
-	sess := contextSessionByteCounter(ctx)
-	if exp == nil && sess == nil {
-		return conn, nil // no point in wrapping
+	if exp := contextExperimentByteCounter(ctx); exp != nil {
+		conn = &bytecounter.Conn{Conn: conn, Counter: exp}
 	}
-	return &byteCounterConnWrapper{Conn: conn, exp: exp, sess: sess}, nil
+	if sess := contextSessionByteCounter(ctx); sess != nil {
+		conn = &bytecounter.Conn{Conn: conn, Counter: sess}
+	}
+	return conn, nil
 }
 
 type byteCounterSessionKey struct{}
@@ -53,31 +54,3 @@ func contextExperimentByteCounter(ctx context.Context) *bytecounter.Counter {
 func WithExperimentByteCounter(ctx context.Context, counter *bytecounter.Counter) context.Context {
 	return context.WithValue(ctx, byteCounterExperimentKey{}, counter)
 }
-
-type byteCounterConnWrapper struct {
-	net.Conn
-	exp  *bytecounter.Counter
-	sess *bytecounter.Counter
-}
-
-func (c *byteCounterConnWrapper) Read(p []byte) (int, error) {
-	count, err := c.Conn.Read(p)
-	if c.exp != nil {
-		c.exp.CountBytesReceived(count)
-	}
-	if c.sess != nil {
-		c.sess.CountBytesReceived(count)
-	}
-	return count, err
-}
-
-func (c *byteCounterConnWrapper) Write(p []byte) (int, error) {
-	count, err := c.Conn.Write(p)
-	if c.exp != nil {
-		c.exp.CountBytesSent(count)
-	}
-	if c.sess != nil {
-		c.sess.CountBytesSent(count)
-	}
-	return count, err
-}

internal/engine/netx/dialer/bytecounter_test.go

@@ -48,9 +48,15 @@ func TestByteCounterNormalUsage(t *testing.T) {
 	if err := dorequest(ctx, "http://facebook.com"); err != nil {
 		t.Fatal(err)
 	}
+	if exp.Received.Load() <= 0 {
+		t.Fatal("experiment should have received some bytes")
+	}
 	if sess.Received.Load() <= exp.Received.Load() {
 		t.Fatal("session should have received more than experiment")
 	}
+	if exp.Sent.Load() <= 0 {
+		t.Fatal("experiment should have sent some bytes")
+	}
 	if sess.Sent.Load() <= exp.Sent.Load() {
 		t.Fatal("session should have sent more than experiment")
 	}