mvcc: Add metric for count of db key revisions compacted.

When digging into etcd/boltdb "storage space exceeded" issues, this metric may help answer questions about if/when compactions occured and how much data was freed.

mvcc/kvstore_compaction.go

@@ -22,6 +22,8 @@ import (
 func (s *store) scheduleCompaction(compactMainRev int64, keep map[revision]struct{}) bool {
 	totalStart := time.Now()
 	defer dbCompactionTotalDurations.Observe(float64(time.Since(totalStart) / time.Millisecond))
+	keyCompactions := 0
+	defer func() { dbCompactionKeysCounter.Add(float64(keyCompactions)) }()
 
 	end := make([]byte, 8)
 	binary.BigEndian.PutUint64(end, uint64(compactMainRev+1))
@@ -40,6 +42,7 @@ func (s *store) scheduleCompaction(compactMainRev int64, keep map[revision]struc
 			rev = bytesToRev(key)
 			if _, ok := keep[rev]; !ok {
 				tx.UnsafeDelete(keyBucketName, key)
+				keyCompactions++
 			}
 		}
 

mvcc/metrics.go

@@ -131,6 +131,14 @@ var (
 			Buckets: prometheus.ExponentialBuckets(100, 2, 14),
 		})
 
+	dbCompactionKeysCounter = prometheus.NewCounter(
+		prometheus.CounterOpts{
+			Namespace: "etcd_debugging",
+			Subsystem: "mvcc",
+			Name:      "db_compaction_keys_total",
+			Help:      "Total number of db keys compacted.",
+		})
+
 	dbTotalSize = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
 		Namespace: "etcd_debugging",
 		Subsystem: "mvcc",
@@ -162,6 +170,7 @@ func init() {
 	prometheus.MustRegister(indexCompactionPauseDurations)
 	prometheus.MustRegister(dbCompactionPauseDurations)
 	prometheus.MustRegister(dbCompactionTotalDurations)
+	prometheus.MustRegister(dbCompactionKeysCounter)
 	prometheus.MustRegister(dbTotalSize)
 }