Add a way to check whether a given monitor has been registered.

The JMX Registry will return true if any of the names for the MBeans that would be created for a given Monitor has been registered.
Because of the way composite monitors work there could be a monitor that is not directly registered. Is the intent to return true only if an instance with the config is directly registered?

Could the signature be changed to allow checking based only on the config or do we need the actual monitor?