loki/wal: fix panic for duplicate registered metrics (#5705)

Signed-off-by: Paschalis Tsilias <[email protected]>

CHANGELOG.md

@@ -98,6 +98,9 @@ Main (unreleased)
 
 - Fixed a bug where UDP syslog messages were never processed (@joshuapare)
 
+- Fix a bug where reloading the configuration of a `loki.write` component lead
+  to a panic. (@tpaschalis)
+
 ### Enhancements
 
 - The `loki.write` WAL now has snappy compression enabled by default. (@thepalbi)

component/common/loki/wal/watcher_metrics.go

@@ -80,13 +80,23 @@ func NewWatcherMetrics(reg prometheus.Registerer) *WatcherMetrics {
 	}
 
 	if reg != nil {
-		reg.MustRegister(m.recordsRead)
-		reg.MustRegister(m.recordDecodeFails)
-		reg.MustRegister(m.droppedWriteNotifications)
-		reg.MustRegister(m.segmentRead)
-		reg.MustRegister(m.currentSegment)
-		reg.MustRegister(m.watchersRunning)
+		m.recordsRead = mustRegisterOrGet(reg, m.recordsRead).(*prometheus.CounterVec)
+		m.recordDecodeFails = mustRegisterOrGet(reg, m.recordDecodeFails).(*prometheus.CounterVec)
+		m.droppedWriteNotifications = mustRegisterOrGet(reg, m.droppedWriteNotifications).(*prometheus.CounterVec)
+		m.segmentRead = mustRegisterOrGet(reg, m.segmentRead).(*prometheus.CounterVec)
+		m.currentSegment = mustRegisterOrGet(reg, m.currentSegment).(*prometheus.GaugeVec)
+		m.watchersRunning = mustRegisterOrGet(reg, m.watchersRunning).(*prometheus.GaugeVec)
 	}
 
 	return m
 }
+
+func mustRegisterOrGet(reg prometheus.Registerer, c prometheus.Collector) prometheus.Collector {
+	if err := reg.Register(c); err != nil {
+		if are, ok := err.(prometheus.AlreadyRegisteredError); ok {
+			return are.ExistingCollector
+		}
+		panic(err)
+	}
+	return c
+}