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 ""