fix(metrics): log collector registration failures

Registration errors were discarded, leaving the collector silently
absent from the metrics endpoint while the vectors still recorded.
Duplicate registrations remain ignored; they are expected when several
instances are constructed in the same process.

pkg/metrics/metrics.go

@@ -1,11 +1,13 @@
 package metrics
 
 import (
+	"errors"
 	"net/url"
 	"strings"
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
+	log "github.com/sirupsen/logrus"
 )
 
 const (
@@ -131,6 +133,19 @@ func Register() {
 	)
 }
 
+func RegisterCollector(collector prometheus.Collector) {
+	err := prometheus.DefaultRegisterer.Register(collector)
+	if err == nil {
+		return
+	}
+
+	if _, ok := errors.AsType[prometheus.AlreadyRegisteredError](err); ok {
+		return
+	}
+
+	log.Warnf("metrics: registering collector: %+v", err)
+}
+
 func ObserveRedisLatency(operation string, fun func() error) error {
 	timer := time.Now()
 	err := fun()

pkg/middleware/prometheus.go

@@ -56,8 +56,8 @@ func Prometheus(provider string, buckets ...float64) *PrometheusMiddleware {
 		[]string{"code", "method", "path", "host"},
 	)
 
-	prometheus.Register(m.reqs)
-	prometheus.Register(m.latency)
+	metrics.RegisterCollector(m.reqs)
+	metrics.RegisterCollector(m.latency)
 
 	return &m
 }

pkg/session/store.go

@@ -5,12 +5,12 @@ import (
 	"fmt"
 	"time"
 
-	"github.com/prometheus/client_golang/prometheus"
 	"github.com/redis/go-redis/extra/redisotel/v9"
 	"github.com/redis/go-redis/extra/redisprometheus/v9"
 	log "github.com/sirupsen/logrus"
 
 	"github.com/nais/wonderwall/pkg/config"
+	"github.com/nais/wonderwall/pkg/metrics"
 )
 
 type Store interface {
@@ -34,7 +34,7 @@ func NewStore(cfg *config.Config) (Store, error) {
 	}
 
 	collector := redisprometheus.NewCollector("wonderwall", "", redisClient)
-	prometheus.Register(collector)
+	metrics.RegisterCollector(collector)
 
 	ctx, cancel := context.WithTimeout(context.Background(), time.Second*30)
 	defer cancel()