fix(metrics): skip connection wrapping when metrics are disabled

When xmetrics.IsEnabled() is false (no -api / no metrics.addr configured),
WrapListener now returns the listener unchanged instead of wrapping every
accepted connection in a metrics.serverConn. This allows bare *net.TCPConn
to reach the forwarding layer, enabling future splice(2) optimization.

metrics/wrapper/listener.go

@@ -2,6 +2,8 @@ package wrapper
 
 import (
 	"net"
+
+	xmetrics "github.com/go-gost/x/metrics"
 )
 
 type listener struct {
@@ -10,10 +12,11 @@ type listener struct {
 }
 
 // WrapListener wraps a net.Listener so that all accepted connections are
-// automatically wrapped with WrapConn for metrics tracking. If ln is nil,
+// automatically wrapped with WrapConn for metrics tracking. If metrics are
-// nil is returned.
+// not enabled, ln is returned unchanged to allow splice(2) optimization
+// on bare *net.TCPConn. If ln is nil, nil is returned.
 func WrapListener(service string, ln net.Listener) net.Listener {
-	if ln == nil {
+	if ln == nil || !xmetrics.IsEnabled() {
 		return ln
 	}
 	return &listener{