test(metrics/wrapper): enable metrics where wrapping is asserted

Since WrapListener returns the listener unchanged when metrics are disabled (3f01456, kept for the splice(2) optimization), the accept-time wrapping test never saw a wrapped conn and failed on the type assertion. Enable metrics for the test with a helper that restores the previous state, and add a companion test that pins the disabled passthrough behavior.

metrics/wrapper/listener_test.go

@@ -4,8 +4,20 @@ import (
 	"errors"
 	"net"
 	"testing"
+
+	xmetrics "github.com/go-gost/x/metrics"
 )
 
+// enableMetrics turns metrics on for the duration of the test and restores
+// the previous state afterwards. WrapListener skips connection wrapping when
+// metrics are disabled, so tests that observe wrapped conns must enable them.
+func enableMetrics(t *testing.T) {
+	t.Helper()
+	prev := xmetrics.IsEnabled()
+	xmetrics.Enable(true)
+	t.Cleanup(func() { xmetrics.Enable(prev) })
+}
+
 func TestWrapListenerNil(t *testing.T) {
 	if ln := WrapListener("svc", nil); ln != nil {
 		t.Error("WrapListener(nil) should return nil")
@@ -22,6 +34,8 @@ func (l *mockListener) Close() error              { return nil }
 func (l *mockListener) Addr() net.Addr            { return &net.TCPAddr{IP: net.IPv4(127, 0, 0, 1), Port: 8080} }
 
 func TestWrapListenerAccept(t *testing.T) {
+	enableMetrics(t)
+
 	client, server := net.Pipe()
 
 	ln := &mockListener{
@@ -68,6 +82,39 @@ func TestWrapListenerAcceptError(t *testing.T) {
 	}
 }
 
+// TestWrapListenerMetricsDisabled documents the splice(2) optimization: with
+// metrics disabled the listener is returned unchanged and accepted
+// connections are not wrapped, leaving bare *net.TCPConn for the
+// forwarding layer.
+func TestWrapListenerMetricsDisabled(t *testing.T) {
+	prev := xmetrics.IsEnabled()
+	xmetrics.Enable(false)
+	t.Cleanup(func() { xmetrics.Enable(prev) })
+
+	client, server := net.Pipe()
+	defer client.Close()
+
+	ln := &mockListener{
+		accept: func() (net.Conn, error) {
+			return server, nil
+		},
+	}
+
+	wrapped := WrapListener("test-svc", ln)
+	if wrapped != net.Listener(ln) {
+		t.Fatalf("WrapListener returned %T, want the listener unchanged when metrics are disabled", wrapped)
+	}
+
+	c, err := wrapped.Accept()
+	if err != nil {
+		t.Fatalf("Accept error: %v", err)
+	}
+	if _, ok := c.(*serverConn); ok {
+		t.Error("accepted connection should not be wrapped when metrics are disabled")
+	}
+	c.Close()
+}
+
 func TestWrapListenerAddr(t *testing.T) {
 	ln := &mockListener{
 		accept: func() (net.Conn, error) { return nil, errors.New("not used") },