[jmx-scraper] Add support for Zookeeper target system

[atoulme]

Add support for Zookeeper with the jmx-scraper as a new target system.

[SylvainJuge]

Do you have any suggested zookeeper MBean attributes to capture as metrics to support ?

[atoulme]

This page describes the MBeans they expose, but not their attributes sadly. An existing integration for Zookeeper exists with a direct connection over commands: https://zookeeper.apache.org/doc/r3.4.8/zookeeperAdmin.html#sc_zkCommands

https://zookeeper.apache.org/doc/r3.9.3/zookeeperJMX.html

Anything under the ZooKeeperServer MBean (be it Leader or Follower) would help:
https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/server/ZooKeeperServer.java#L105
esp the ServerStats:
https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/server/ServerStats.java#L31