feat(go): track gateway active connections

go/vehicle-gateway/internal/metrics/metrics.go

@@ -13,10 +13,14 @@ type Labels map[string]string
 type Registry struct {
 	mu       sync.RWMutex
 	counters map[string]map[string]float64
+	gauges   map[string]map[string]float64
 }
 
 func NewRegistry() *Registry {
-	return &Registry{counters: map[string]map[string]float64{}}
+	return &Registry{
+		counters: map[string]map[string]float64{},
+		gauges:   map[string]map[string]float64{},
+	}
 }
 
 func (r *Registry) IncCounter(name string, labels Labels) {
@@ -37,6 +41,34 @@ func (r *Registry) AddCounter(name string, labels Labels, value float64) {
 	r.counters[name][key] += value
 }
 
+func (r *Registry) SetGauge(name string, labels Labels, value float64) {
+	name = strings.TrimSpace(name)
+	if r == nil || name == "" {
+		return
+	}
+	key := labelsKey(labels)
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.gauges[name] == nil {
+		r.gauges[name] = map[string]float64{}
+	}
+	r.gauges[name][key] = value
+}
+
+func (r *Registry) AddGauge(name string, labels Labels, value float64) {
+	name = strings.TrimSpace(name)
+	if r == nil || name == "" || value == 0 {
+		return
+	}
+	key := labelsKey(labels)
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.gauges[name] == nil {
+		r.gauges[name] = map[string]float64{}
+	}
+	r.gauges[name][key] += value
+}
+
 func (r *Registry) Render() string {
 	if r == nil {
 		return ""
@@ -52,29 +84,44 @@ func (r *Registry) Render() string {
 
 	var b strings.Builder
 	for _, name := range names {
-		b.WriteString("# TYPE ")
-		b.WriteString(name)
-		b.WriteString(" counter\n")
-		var keys []string
-		for key := range r.counters[name] {
-			keys = append(keys, key)
-		}
-		sort.Strings(keys)
-		for _, key := range keys {
-			b.WriteString(name)
-			if key != "" {
-				b.WriteString("{")
-				b.WriteString(key)
-				b.WriteString("}")
-			}
-			b.WriteString(" ")
-			b.WriteString(formatNumber(r.counters[name][key]))
-			b.WriteString("\n")
-		}
+		renderMetricFamily(&b, name, "counter", r.counters[name])
+	}
+
+	names = names[:0]
+	for name := range r.gauges {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		renderMetricFamily(&b, name, "gauge", r.gauges[name])
 	}
 	return b.String()
 }
 
+func renderMetricFamily(b *strings.Builder, name string, metricType string, values map[string]float64) {
+	b.WriteString("# TYPE ")
+	b.WriteString(name)
+	b.WriteString(" ")
+	b.WriteString(metricType)
+	b.WriteString("\n")
+	var keys []string
+	for key := range values {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+	for _, key := range keys {
+		b.WriteString(name)
+		if key != "" {
+			b.WriteString("{")
+			b.WriteString(key)
+			b.WriteString("}")
+		}
+		b.WriteString(" ")
+		b.WriteString(formatNumber(values[key]))
+		b.WriteString("\n")
+	}
+}
+
 func NewHandler(registry *Registry) http.Handler {
 	if registry == nil {
 		registry = NewRegistry()

go/vehicle-gateway/internal/metrics/metrics_test.go

@@ -27,6 +27,25 @@ func TestRegistryRendersCountersWithLabels(t *testing.T) {
 	}
 }
 
+func TestRegistryRendersGaugesWithLabels(t *testing.T) {
+	registry := NewRegistry()
+	registry.SetGauge("vehicle_gateway_connections", Labels{"protocol": "JT808"}, 2)
+	registry.AddGauge("vehicle_gateway_connections", Labels{"protocol": "JT808"}, -1)
+	registry.SetGauge("vehicle_gateway_connections", Labels{"protocol": "GB32960"}, 3)
+
+	text := registry.Render()
+
+	for _, want := range []string{
+		`# TYPE vehicle_gateway_connections gauge`,
+		`vehicle_gateway_connections{protocol="GB32960"} 3`,
+		`vehicle_gateway_connections{protocol="JT808"} 1`,
+	} {
+		if !strings.Contains(text, want) {
+			t.Fatalf("metrics missing %s:\n%s", want, text)
+		}
+	}
+}
+
 func TestHandlerServesPrometheusText(t *testing.T) {
 	registry := NewRegistry()
 	registry.IncCounter("vehicle_kafka_commits_total", Labels{"service": "history"})