feat(go): expose kafka consumer lag metrics

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

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"net/http"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 )
@@ -69,6 +70,17 @@ func (r *Registry) AddGauge(name string, labels Labels, value float64) {
 	r.gauges[name][key] += value
 }
 
+func (r *Registry) SetKafkaLag(name string, topic string, partition int, offset int64, highWaterMark int64) {
+	lag := highWaterMark - offset - 1
+	if lag < 0 {
+		lag = 0
+	}
+	r.SetGauge(name, Labels{
+		"topic":     topic,
+		"partition": strconv.Itoa(partition),
+	}, float64(lag))
+}
+
 func (r *Registry) Render() string {
 	if r == nil {
 		return ""

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

@@ -46,6 +46,24 @@ func TestRegistryRendersGaugesWithLabels(t *testing.T) {
 	}
 }
 
+func TestRegistryRecordsKafkaLagGauge(t *testing.T) {
+	registry := NewRegistry()
+
+	registry.SetKafkaLag("vehicle_history_kafka_lag", "vehicle.raw.go.jt808.v1", 3, 101, 130)
+	registry.SetKafkaLag("vehicle_history_kafka_lag", "vehicle.raw.go.gb32960.v1", 0, 9, 9)
+
+	text := registry.Render()
+	for _, want := range []string{
+		`# TYPE vehicle_history_kafka_lag gauge`,
+		`vehicle_history_kafka_lag{partition="0",topic="vehicle.raw.go.gb32960.v1"} 0`,
+		`vehicle_history_kafka_lag{partition="3",topic="vehicle.raw.go.jt808.v1"} 28`,
+	} {
+		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"})