feat(go): expose kafka consumer lag metrics

go/vehicle-gateway/cmd/realtime-api/main.go

@@ -218,6 +218,7 @@ func processRealtimeMessage(ctx context.Context, logger interface {
 	defer cancel()
 
 	addRealtimeMetric(registry, "vehicle_realtime_kafka_messages_total", message, "received")
+	addRealtimeLagMetric(registry, message)
 	var env envelope.FrameEnvelope
 	if err := json.Unmarshal(message.Value, &env); err != nil {
 		addRealtimeMetric(registry, "vehicle_realtime_kafka_messages_total", message, "invalid_json")
@@ -249,6 +250,13 @@ func addRealtimeMetric(registry *metrics.Registry, name string, message kafka.Me
 	})
 }
 
+func addRealtimeLagMetric(registry *metrics.Registry, message kafka.Message) {
+	if registry == nil {
+		return
+	}
+	registry.SetKafkaLag("vehicle_realtime_kafka_lag", message.Topic, message.Partition, message.Offset, message.HighWaterMark)
+}
+
 func env(key string, fallback string) string {
 	value := strings.TrimSpace(os.Getenv(key))
 	if value == "" {