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/prom_test.go

@@ -0,0 +1,114 @@
+package metrics
+
+import (
+	"testing"
+
+	"github.com/go-gost/core/metrics"
+)
+
+func TestPromMetricsGaugeHostLabel(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	// MetricServicesGauge: labels=["host"], host auto-added.
+	g := GetGauge(MetricServicesGauge, nil)
+	if g == nil {
+		t.Fatal("gauge should not be nil")
+	}
+	g.Set(3)
+	g.Add(1)
+	g.Inc()
+	g.Dec()
+}
+
+func TestPromMetricsCounterKnownNames(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	tests := []struct {
+		name   metrics.MetricName
+		labels metrics.Labels
+	}{
+		{MetricServiceRequestsCounter, metrics.Labels{"service": "s", "client": "c"}},
+		{MetricServiceTransferInputBytesCounter, metrics.Labels{"service": "s", "client": "c"}},
+		{MetricServiceTransferOutputBytesCounter, metrics.Labels{"service": "s", "client": "c"}},
+		{MetricServiceHandlerErrorsCounter, metrics.Labels{"service": "s", "client": "c"}},
+		{MetricChainErrorsCounter, metrics.Labels{"chain": "ch", "node": "n"}},
+		{MetricRecorderRecordsCounter, metrics.Labels{"recorder": "r"}},
+	}
+
+	for _, tt := range tests {
+		c := GetCounter(tt.name, tt.labels)
+		if c == nil {
+			t.Errorf("GetCounter(%q) returned nil", tt.name)
+			continue
+		}
+		c.Inc()
+		c.Add(10)
+	}
+}
+
+func TestPromMetricsGaugeKnownNames(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	g := GetGauge(MetricServiceRequestsInFlightGauge, metrics.Labels{"service": "s", "client": "c"})
+	if g == nil {
+		t.Fatal("gauge should not be nil")
+	}
+	g.Set(1)
+	g.Inc()
+	g.Dec()
+}
+
+func TestPromMetricsObserverKnownNames(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	tests := []struct {
+		name   metrics.MetricName
+		labels metrics.Labels
+	}{
+		{MetricServiceRequestsDurationObserver, metrics.Labels{"service": "s"}},
+		{MetricNodeConnectDurationObserver, metrics.Labels{"chain": "ch", "node": "n"}},
+	}
+
+	for _, tt := range tests {
+		o := GetObserver(tt.name, tt.labels)
+		if o == nil {
+			t.Errorf("GetObserver(%q) returned nil", tt.name)
+			continue
+		}
+		o.Observe(0.5)
+		o.Observe(1.0)
+	}
+}
+
+func TestPromMetricsUnknownNameReturnsNoop(t *testing.T) {
+	orig := IsEnabled()
+	Enable(true)
+	defer Enable(orig)
+
+	// Unknown metric names should return noop implementations (non-nil, safe to call).
+	c := GetCounter("unknown_counter", nil)
+	if c == nil {
+		t.Error("counter for unknown name should not be nil")
+	}
+	c.Inc()
+
+	g := GetGauge("unknown_gauge", nil)
+	if g == nil {
+		t.Error("gauge for unknown name should not be nil")
+	}
+	g.Set(1)
+
+	o := GetObserver("unknown_observer", nil)
+	if o == nil {
+		t.Error("observer for unknown name should not be nil")
+	}
+	o.Observe(0.1)
+}