ZOOKEEPER-4398: Add configurable Prometheus metric name prefix.

[skidder]

Resolves ZOOKEEPER-4398 by adding an optional metric configuration property for a string prefix for all Zookeeper metric names.

It's common to have Prometheus metrics scoped to a namespace by using the application name as its prefix. For instance, Apache Kafka metrics have the prefix of kafka_, making them easier to identify.

This new config could be used to namespace Zookeeper metrics (e.g. zookeeper_up instead of simply up). The default behavior does not use a prefix.

[hicwic]

i need this to be merged too. Metrics in this state/name are unbrowsable/unusable.