HDFS-11211. Add a time unit to the DataNode client trace format. Contributed by Jagadesh Kiran N

(cherry picked from commit 43cb1678cc)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java

@@ -278,7 +278,7 @@ public class DataNode extends ReconfigurableBase
         ", offset: %s" + // offset
         ", srvID: %s" +  // DatanodeRegistration
         ", blockid: %s" + // block id
-        ", duration: %s";  // duration time
+        ", duration(ns): %s";  // duration time
         
   static final Log ClientTraceLog =
     LogFactory.getLog(DataNode.class.getName() + ".clienttrace");