fix(metrics): add doc comments, fix labels mutation and nil returns, add idempotent Close, add nil guards, add tests

- Fix promMetrics.Gauge/Counter/Observer mutating caller's labels map by
  using maps.Copy instead of direct assignment
- Fix Gauge/Counter/Observer returning nil for unknown metric names
  (now return noop implementations)
- Fix metricService.Close() race with sync.Once for safe concurrent calls
- Fix SetDSCP returning nil instead of errUnsupport on unsupported ops
- Add nil guards to WrapConn, WrapPacketConn, WrapUDPConn, WrapListener
- Add doc comments to all exported symbols
- Add 45 unit tests (metrics, noop, prom, service, wrapper)

metrics/noop_test.go

@@ -0,0 +1,80 @@
+package metrics
+
+import (
+	"testing"
+
+	"github.com/go-gost/core/metrics"
+)
+
+func TestNoopReturnsSingleton(t *testing.T) {
+	n1 := Noop()
+	n2 := Noop()
+	if n1 != n2 {
+		t.Error("Noop() should return the same instance every time")
+	}
+}
+
+func TestNoopMetricsCounter(t *testing.T) {
+	c := Noop().Counter("any", nil)
+	if c == nil {
+		t.Fatal("noop Counter should not be nil")
+	}
+	c.Inc()
+	c.Add(42)
+}
+
+func TestNoopMetricsGauge(t *testing.T) {
+	g := Noop().Gauge("any", nil)
+	if g == nil {
+		t.Fatal("noop Gauge should not be nil")
+	}
+	g.Inc()
+	g.Dec()
+	g.Add(1.5)
+	g.Set(100)
+}
+
+func TestNoopMetricsObserver(t *testing.T) {
+	o := Noop().Observer("any", nil)
+	if o == nil {
+		t.Fatal("noop Observer should not be nil")
+	}
+	o.Observe(0.5)
+}
+
+func TestNoopMetricsSameInstances(t *testing.T) {
+	m := Noop()
+	// Each call returns the shared singleton noop instances
+	if m.Counter("a", nil) != m.Counter("b", nil) {
+		t.Error("noop Counter should return same instance regardless of name")
+	}
+	if m.Gauge("a", nil) != m.Gauge("b", nil) {
+		t.Error("noop Gauge should return same instance regardless of name")
+	}
+	if m.Observer("a", nil) != m.Observer("b", nil) {
+		t.Error("noop Observer should return same instance regardless of name")
+	}
+}
+
+func TestNoopPackageVarsAreNoopTypes(t *testing.T) {
+	// The package-level noop variables should be instances of the concrete noop types.
+	// These are the values returned by GetCounter/Gauge/Observer when disabled.
+	c := Noop().Counter("x", metrics.Labels{"k": "v"})
+	if c == nil {
+		t.Fatal("noop Counter from Noop() should not be nil")
+	}
+	c.Inc()
+	c.Add(10)
+
+	g := Noop().Gauge("x", metrics.Labels{"k": "v"})
+	if g == nil {
+		t.Fatal("noop Gauge from Noop() should not be nil")
+	}
+	g.Set(5)
+
+	o := Noop().Observer("x", metrics.Labels{"k": "v"})
+	if o == nil {
+		t.Fatal("noop Observer from Noop() should not be nil")
+	}
+	o.Observe(0.1)
+}