Make E2E service accept a prometheus registerer

I was informed about a usecase that uses KMinions  end to end service
as a library. In order to provide more control about the exported
metrics the E2E service now accepts a Prometheus Gatherer that is used to
register all exposed metrics

e2e/service.go

@@ -8,7 +8,6 @@ import (
 	"github.com/cloudhut/kminion/v2/kafka"
 	"github.com/google/uuid"
 	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promauto"
 	"github.com/twmb/franz-go/pkg/kgo"
 	"go.uber.org/zap"
 )
@@ -44,7 +43,7 @@ type Service struct {
 }
 
 // NewService creates a new instance of the e2e moinitoring service (wow)
-func NewService(ctx context.Context, cfg Config, logger *zap.Logger, kafkaSvc *kafka.Service, metricNamespace string) (*Service, error) {
+func NewService(ctx context.Context, cfg Config, logger *zap.Logger, kafkaSvc *kafka.Service, promRegisterer prometheus.Registerer) (*Service, error) {
 	minionID := uuid.NewString()
 	groupID := fmt.Sprintf("%v-%v", cfg.Consumer.GroupIdPrefix, minionID)
 
@@ -94,29 +93,32 @@ func NewService(ctx context.Context, cfg Config, logger *zap.Logger, kafkaSvc *k
 	svc.messageTracker = newMessageTracker(svc)
 
 	makeCounterVec := func(name string, labelNames []string, help string) *prometheus.CounterVec {
-		return promauto.NewCounterVec(prometheus.CounterOpts{
-			Namespace: metricNamespace,
+		cv := prometheus.NewCounterVec(prometheus.CounterOpts{
 			Subsystem: "end_to_end",
 			Name:      name,
 			Help:      help,
 		}, labelNames)
+		promRegisterer.MustRegister(cv)
+		return cv
 	}
 	makeGaugeVec := func(name string, labelNames []string, help string) *prometheus.GaugeVec {
-		return promauto.NewGaugeVec(prometheus.GaugeOpts{
-			Namespace: metricNamespace,
+		gv := prometheus.NewGaugeVec(prometheus.GaugeOpts{
 			Subsystem: "end_to_end",
 			Name:      name,
 			Help:      help,
 		}, labelNames)
+		promRegisterer.MustRegister(gv)
+		return gv
 	}
 	makeHistogramVec := func(name string, maxLatency time.Duration, labelNames []string, help string) *prometheus.HistogramVec {
-		return promauto.NewHistogramVec(prometheus.HistogramOpts{
-			Namespace: metricNamespace,
+		hv := prometheus.NewHistogramVec(prometheus.HistogramOpts{
 			Subsystem: "end_to_end",
 			Name:      name,
 			Help:      help,
 			Buckets:   createHistogramBuckets(maxLatency),
 		}, labelNames)
+		promRegisterer.MustRegister(hv)
+		return hv
 	}
 
 	// Low-level info
@@ -178,7 +180,6 @@ func (s *Service) Start(ctx context.Context) error {
 		case <-initCh:
 			isInitialized = true
 			s.logger.Info("consumer has been successfully initialized")
-			break
 		case <-ctx.Done():
 			return nil
 		}

main.go

@@ -54,6 +54,8 @@ func main() {
 		}
 	}()
 
+	wrappedRegisterer := promclient.WrapRegistererWithPrefix(cfg.Exporter.Namespace+"_", promclient.DefaultRegisterer)
+
 	// Create kafka service
 	kafkaSvc := kafka.NewService(cfg.Kafka, logger)
 
@@ -77,7 +79,7 @@ func main() {
 			cfg.Minion.EndToEnd,
 			logger,
 			kafkaSvc,
-			cfg.Exporter.Namespace,
+			wrappedRegisterer,
 		)
 		if err != nil {
 			logger.Fatal("failed to create end-to-end monitoring service: %w", zap.Error(err))