org.apache.hadoop.metrics2.util.MBeans.register is a utility function to register objects to the JMX registry with a given name prefix and name.
JMX supports any additional key value pairs which could be part the the address of the jmx bean. For example: java.lang:type=MemoryManager,name=CodeCacheManager
Using this method we can query a group of mbeans, for example we can add the same tag to similar mbeans from namenode and datanode.
This patch adds a small modification to support custom key value pairs and also introduce a new unit test for MBeans utility which was missing until now.