Formalize the names of the metrics included in the metric registry.

It's needed to refactor metric names to obey the global rule - all of them must start with name of the registry they belongs to. And add corresponding assertion.

The purpose of this change is to make all metrics that belongs to one MetricRegistry have the name that obeys mentioned above rule regardless how they were registered. It helps to use a consistent approach to work with all metrics.

The following metric names will be changed to obey this rule:

JoinedNodes -> io.discovery.JoinedNodes
LeftNodes -> io.discovery.LeftNodes
FailedNodes -> io.discovery.FailedNodes
PendingMessagesRegistered -> io.discovery.PendingMessagesRegistered
CommunicationErrors -> io.discovery.CommunicationErrors
CurrentTopologyVersion -> io.discovery.CurrentTopologyVersion