loosen type requirements for JmxAttributeGauge constructor

Previously an MBeanServer was required to create a JmxAttributeGauge object
even though instances of JmxAttributeGauge only use methods from the
MBeanServer's super-interface, MBeanServerConnection. Relaxing this constraint
allows passing a JmxAttributeGauge any MBeanServerConnection (e.g. the
connection returned by JMXConnector.getMBeanServerConnection).