fix: duplicate metrics collector registration attempted panic

caddy/caddy_test.go

@@ -52,7 +52,19 @@ localhost:9080 {
 		mercure {
 			anonymous
 			publisher_jwt !ChangeMe!
-			%s
+			%[1]s
+		}
+
+		respond 404
+	}
+}
+
+example.com:9080 {
+	route {
+		mercure {
+			anonymous
+			publisher_jwt !ChangeMe!
+			%[1]s
 		}
 
 		respond 404

metrics.go

@@ -1,6 +1,7 @@
 package mercure
 
 import (
+	"errors"
 	"net/http"
 
 	"github.com/dunglas/mercure/common"
@@ -62,9 +63,19 @@ func NewPrometheusMetrics(registry prometheus.Registerer) *PrometheusMetrics {
 		),
 	}
 
-	m.registry.MustRegister(m.subscribers)
-	m.registry.MustRegister(m.subscribersTotal)
-	m.registry.MustRegister(m.updatesTotal)
+	// https://github.com/caddyserver/caddy/pull/6820
+	if err := m.registry.Register(m.subscribers); err != nil &&
+		!errors.As(err, &prometheus.AlreadyRegisteredError{}) {
+		panic(err)
+	}
+	if err := m.registry.Register(m.subscribersTotal); err != nil &&
+		!errors.As(err, &prometheus.AlreadyRegisteredError{}) {
+		panic(err)
+	}
+	if err := m.registry.Register(m.updatesTotal); err != nil &&
+		!errors.As(err, &prometheus.AlreadyRegisteredError{}) {
+		panic(err)
+	}
 
 	return m
 }