HDFS-7449. Add metrics to NFS gateway. Contributed by Brandon Li

(cherry picked from commit f6f2a3f1c73266bfedd802eacde60d8b19b81015)
HADOOP-11389. Clean up byte to string encoding issues in hadoop-common. Contributed by Haohui Mai.
HDFS-7497. Inconsistent report of decommissioning DataNodes between dfsadmin and NameNode webui. Contributed by Yongjun Zhang.

(cherry picked from commit b437f5eef40874287d4fbf9d8e43f1a857b5621f)