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/metrics_test.go

@@ -0,0 +1,173 @@
+package metrics
+
+import (
+	"testing"
+
+	"github.com/go-gost/core/metrics"
+)
+
+func TestEnableDisable(t *testing.T) {
+	// Save and restore original state.
+	orig := IsEnabled()
+	defer Enable(orig)
+
+	Enable(false)
+	if IsEnabled() {
+		t.Error("IsEnabled should return false after Enable(false)")
+	}
+
+	Enable(true)
+	if !IsEnabled() {
+		t.Error("IsEnabled should return true after Enable(true)")
+	}
+
+	Enable(false)
+	if IsEnabled() {
+		t.Error("IsEnabled should return false after second Enable(false)")
+	}
+}
+
+func TestGetCounterDisabled(t *testing.T) {
+	orig := IsEnabled()
+	Enable(false)
+	defer Enable(orig)
+
+	c := GetCounter(MetricServiceRequestsCounter, metrics.Labels{"service": "s", "client": "c"})
+	if c == nil {
+		t.Error("GetCounter should not return nil when disabled")
+	}
+	// Should be noop — calling methods must not panic.
+	c.Inc()
+	c.Add(42)
+	c.Add(-1)
+}
+
+func TestGetGaugeDisabled(t *testing.T) {
+	orig := IsEnabled()
+	Enable(false)
+	defer Enable(orig)
+
+	// MetricServicesGauge has only ["host"] label (auto-added), so pass no extra labels.
+	g := GetGauge(MetricServicesGauge, nil)
+	if g == nil {
+		t.Error("GetGauge should not return nil when disabled")
+	}
+	g.Inc()
+	g.Dec()
+	g.Add(1.5)
+	g.Set(100)
+	g.Set(-1)
+}
+
+func TestGetObserverDisabled(t *testing.T) {
+	orig := IsEnabled()
+	Enable(false)
+	defer Enable(orig)
+
+	o := GetObserver(MetricServiceRequestsDurationObserver, metrics.Labels{"service": "s"})
+	if o == nil {
+		t.Error("GetObserver should not return nil when disabled")
+	}
+	o.Observe(0.5)
+	o.Observe(0)
+	o.Observe(-1)
+}
+
+func TestGetCounterEnabled(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	c := GetCounter(MetricServiceRequestsCounter, metrics.Labels{"service": "s", "client": "c"})
+	if c == nil {
+		t.Error("GetCounter should not return nil when enabled")
+	}
+	// Methods should not panic.
+	c.Inc()
+	c.Add(10)
+}
+
+func TestGetGaugeEnabled(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	// MetricServicesGauge has only ["host"] label (auto-added).
+	g := GetGauge(MetricServicesGauge, nil)
+	if g == nil {
+		t.Error("GetGauge should not return nil when enabled")
+	}
+	g.Inc()
+	g.Dec()
+	g.Set(5)
+}
+
+func TestGetObserverEnabled(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	o := GetObserver(MetricServiceRequestsDurationObserver, metrics.Labels{"service": "s"})
+	if o == nil {
+		t.Error("GetObserver should not return nil when enabled")
+	}
+	o.Observe(0.25)
+}
+
+func TestGetCounterEnabledUnknownName(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	c := GetCounter("nonexistent_metric", nil)
+	if c == nil {
+		t.Error("GetCounter should not return nil for unknown metric name")
+	}
+	c.Inc()
+	c.Add(5)
+}
+
+func TestGetGaugeEnabledUnknownName(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	g := GetGauge("nonexistent_metric", nil)
+	if g == nil {
+		t.Error("GetGauge should not return nil for unknown metric name")
+	}
+	g.Inc()
+	g.Set(1)
+}
+
+func TestGetObserverEnabledUnknownName(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	o := GetObserver("nonexistent_metric", nil)
+	if o == nil {
+		t.Error("GetObserver should not return nil for unknown metric name")
+	}
+	o.Observe(1.0)
+}
+
+func TestMetricConstantsNotEmpty(t *testing.T) {
+	names := []metrics.MetricName{
+		MetricServicesGauge,
+		MetricServiceRequestsCounter,
+		MetricServiceRequestsInFlightGauge,
+		MetricServiceRequestsDurationObserver,
+		MetricServiceTransferInputBytesCounter,
+		MetricServiceTransferOutputBytesCounter,
+		MetricNodeConnectDurationObserver,
+		MetricServiceHandlerErrorsCounter,
+		MetricChainErrorsCounter,
+		MetricRecorderRecordsCounter,
+	}
+	for i, n := range names {
+		if n == "" {
+			t.Errorf("metric constant at index %d is empty", i)
+		}
+	}
+}