mvcc: Export a "Last DB compaction" timestamp metric (etcd-io#12176)

This is to aid with debugging the effectiveness of systems that
manually take care of cluster compaction, and have greater visibity
into recent compactions.

It can be handy to alert on the exactly how long it was since a
compaction (and also to put on dashboards) had happened.

---

Tested using a test cluster, the final result looks like this:

```
	root@etcd-1:~# ETCDCTL_API=3 /tmp/test-etcd/etcdctl --endpoints=192.168.232.10:2379 compact 1012
	compacted revision 1012

	root@etcd-1:~# curl -s 192.168.232.10:2379/metrics | grep last
	# HELP etcd_debugging_mvcc_db_compaction_last The unix time since the last db compaction.  Resets to 0 on start.
	# TYPE etcd_debugging_mvcc_db_compaction_last gauge
	etcd_debugging_mvcc_db_compaction_last 1.595873939e+09
```

mvcc/kvstore_compaction.go

@@ -27,6 +27,7 @@ func (s *store) scheduleCompaction(compactMainRev int64, keep map[revision]struc
 	defer func() { dbCompactionTotalMs.Observe(float64(time.Since(totalStart) / time.Millisecond)) }()
 	keyCompactions := 0
 	defer func() { dbCompactionKeysCounter.Add(float64(keyCompactions)) }()
+	defer func() { dbCompactionLast.Set(float64(time.Now().Unix())) }()
 
 	end := make([]byte, 8)
 	binary.BigEndian.PutUint64(end, uint64(compactMainRev+1))

mvcc/metrics.go

@@ -167,6 +167,14 @@ var (
 			Buckets: prometheus.ExponentialBuckets(100, 2, 14),
 		})
 
+	dbCompactionLast = prometheus.NewGauge(
+		prometheus.GaugeOpts{
+			Namespace: "etcd_debugging",
+			Subsystem: "mvcc",
+			Name:      "db_compaction_last",
+			Help:      "The unix time of the last db compaction. Resets to 0 on start.",
+		})
+
 	dbCompactionKeysCounter = prometheus.NewCounter(
 		prometheus.CounterOpts{
 			Namespace: "etcd_debugging",
@@ -324,6 +332,7 @@ func init() {
 	prometheus.MustRegister(indexCompactionPauseMs)
 	prometheus.MustRegister(dbCompactionPauseMs)
 	prometheus.MustRegister(dbCompactionTotalMs)
+	prometheus.MustRegister(dbCompactionLast)
 	prometheus.MustRegister(dbCompactionKeysCounter)
 	prometheus.MustRegister(dbTotalSize)
 	prometheus.MustRegister(dbTotalSizeDebug)