metrics: Allow metrics to be registered multiple times

metrics/database.go

@@ -45,8 +45,8 @@ func NewDefaultDatabaseMetrics(pkg string) DatabaseMetrics {
 			databaseLatencyLabels,
 		),
 	}
-	prometheus.MustRegister(metrics.DatabaseOperations)
-	prometheus.MustRegister(metrics.DatabaseLatencies)
+	metrics.DatabaseOperations = registerOnce(metrics.DatabaseOperations).(*prometheus.CounterVec)
+	metrics.DatabaseLatencies = registerOnce(metrics.DatabaseLatencies).(*prometheus.HistogramVec)
 	return metrics
 }
 

metrics/util.go

@@ -0,0 +1,23 @@
+package metrics
+
+import (
+	"errors"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Registers the collector with Prometheus. If an identical collector is already
+// registered, returns the existing collector, otherwise returns the provided collector.
+// Panics if the collector cannot be registered.
+func registerOnce(collector prometheus.Collector) prometheus.Collector {
+	if err := prometheus.Register(collector); err != nil {
+		are := &prometheus.AlreadyRegisteredError{}
+		if errors.As(err, are) {
+			// Use the old collector from now on.
+			return are.ExistingCollector
+		}
+		// Something else went wrong.
+		panic(err)
+	}
+	return collector
+}