mvcc/backend: fix defrag duration scale

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

mvcc/backend/metrics.go

@@ -37,7 +37,7 @@ var (
 		// 100 MB usually takes 1 sec, so start with 10 MB of 100 ms
 		// lowest bucket start of upper bound 0.1 sec (100 ms) with factor 2
 		// highest bucket start of 0.1 sec * 2^12 == 409.6 sec
-		Buckets: prometheus.ExponentialBuckets(.01, 2, 13),
+		Buckets: prometheus.ExponentialBuckets(.1, 2, 13),
 	})
 
 	snapshotDurations = prometheus.NewHistogram(prometheus.HistogramOpts{