client: add a WithNoProxy dialoption (#3411)

clientconn.go

@@ -194,12 +194,13 @@ func DialContext(ctx context.Context, target string, opts ...DialOption) (conn *
 	cc.mkp = cc.dopts.copts.KeepaliveParams
 
 	if cc.dopts.copts.Dialer == nil {
-		cc.dopts.copts.Dialer = newProxyDialer(
+		cc.dopts.copts.Dialer = func(ctx context.Context, addr string) (net.Conn, error) {
-			func(ctx context.Context, addr string) (net.Conn, error) {
 			network, addr := parseDialTarget(addr)
 			return (&net.Dialer{}).DialContext(ctx, network, addr)
-			},
+		}
-		)
+		if cc.dopts.withProxy {
+			cc.dopts.copts.Dialer = newProxyDialer(cc.dopts.copts.Dialer)
+		}
 	}
 
 	if cc.dopts.copts.UserAgent != "" {

dialoptions.go

@@ -72,6 +72,7 @@ type dialOptions struct {
 	// we need to be able to configure this in tests.
 	resolveNowBackoff func(int) time.Duration
 	resolvers         []resolver.Builder
+	withProxy         bool
 }
 
 // DialOption configures how we set up the connection.
@@ -307,6 +308,16 @@ func WithInsecure() DialOption {
 	})
 }
 
+// WithNoProxy returns a DialOption which disables the use of proxies for this
+// ClientConn. This is ignored if WithDialer or WithContextDialer are used.
+//
+// This API is EXPERIMENTAL.
+func WithNoProxy() DialOption {
+	return newFuncDialOption(func(o *dialOptions) {
+		o.withProxy = false
+	})
+}
+
 // WithTransportCredentials returns a DialOption which configures a connection
 // level security credentials (e.g., TLS/SSL). This should not be used together
 // with WithCredentialsBundle.
@@ -557,6 +568,7 @@ func defaultDialOptions() dialOptions {
 			ReadBufferSize:  defaultReadBufSize,
 		},
 		resolveNowBackoff: internalbackoff.DefaultExponential.Backoff,
+		withProxy:         true,
 	}
 }