HDFS-16526. Addendum Add metrics for slow DataNode (#4191)

hadoop-common-project/hadoop-common/src/site/markdown/Metrics.md

@@ -483,6 +483,7 @@ Each metrics record contains tags such as SessionId and Hostname as additional i
 | `PacketsSlowWriteToOsCache` | Total number of packets whose write to os cache takes more than a certain time (300ms by default) |
 | `slowFlushOrSyncCount` | Total number of packets whose sync/flush takes more than a certain time (300ms by default) |
 | `slowAckToUpstreamCount` | Total number of packets whose upstream ack takes more than a certain time (300ms by default) |
+
 FsVolume
 --------