Refactor getMetricsState method from RollupServiceMBean into a separate ContextMBean interface

Currently, the metrics state is exposed only by RollupServiceMBean, so if rollups are not running within the same context as ingest (i.e we have some blueflood hosts running with rollups turned OFF) we will be unable to use these JMX methods to debug the in-memory shard states. 
This separates out the getMetricsState method into a MBean of its own which gets implemented and loaded by ScheduleContext so that we will be able to access these metrics state irrespective of the rollups being turned ON or OFF.