The ZooKeeper Grafana dashboard is now available

[Justin Ling Mao]

- Due to the work from 
ZOOKEEPER-3447(https://github.com/apache/zookeeper/pull/1008), the ZooKeeper 
Grafana dashboard is now available(https://grafana.com/dashboards/10465) which 
I had included some screenshots for the preview.
- Users can ask for this Grafana dashboard account if having any good 
improvements by writing a email to dev@zookeeper.apache.org.