feat(go): expose stat writer latency histogram

go/vehicle-gateway/cmd/stat-writer/main.go

@@ -83,6 +83,8 @@ type kafkaMessageCommitter interface {
 	CommitMessages(context.Context, ...kafka.Message) error
 }
 
+var statWriteDurationBucketsMS = []float64{1, 5, 10, 25, 50, 100, 250, 500, 1000, 5000}
+
 func processStatMessage(ctx context.Context, logger interface {
 	Error(string, ...any)
 	Warn(string, ...any)
@@ -99,7 +101,10 @@ func processStatMessage(ctx context.Context, logger interface {
 		_ = committer.CommitMessages(messageCtx, message)
 		return
 	}
-	if err := appender.Append(messageCtx, env); err != nil {
+	started := time.Now()
+	err := appender.Append(messageCtx, env)
+	recordStatWriteDuration(registry, message, statusFromError(err), time.Since(started))
+	if err != nil {
 		addStatMetric(registry, "vehicle_stat_writes_total", message, "error")
 		logger.Error("mysql append failed", "topic", message.Topic, "partition", message.Partition, "offset", message.Offset, "event_id", env.StableEventID(), "error", err)
 		return
@@ -127,6 +132,23 @@ func addStatLagMetric(registry *metrics.Registry, message kafka.Message) {
 	registry.SetKafkaLag("vehicle_stat_kafka_lag", message.Topic, message.Partition, message.Offset, message.HighWaterMark)
 }
 
+func recordStatWriteDuration(registry *metrics.Registry, message kafka.Message, status string, elapsed time.Duration) {
+	if registry == nil {
+		return
+	}
+	registry.ObserveHistogram("vehicle_stat_write_duration_ms_histogram", metrics.Labels{
+		"topic":  message.Topic,
+		"status": status,
+	}, statWriteDurationBucketsMS, float64(elapsed.Milliseconds()))
+}
+
+func statusFromError(err error) string {
+	if err != nil {
+		return "error"
+	}
+	return "ok"
+}
+
 type config struct {
 	KafkaBrokers []string
 	KafkaTopics  []string