Add prepare-metric.sh example for engine monitor

[seoeun25]

1. Create topic
   Set KAFKA_HOME, ZK_HOST at prepare-metric.sh and execute it.
   $ bin/prepare-metric.sh

2. Add supervisor
   Set bootstrap.servers at metric/metric-supervisor.json and execute it to overlord.
   $ curl -XPOST -H'Content-Type: application/json' -d @metric/metric-supervisor.json http://localhost:8090/druid/indexer/v1/supervisor