Add a JSON format for existing metrics like this

[chaoyli]

The format is like this.

{
 "tags":
 {
 "metric":"thread_pool",
 "name":"thrift-server-pool",
 "type":"active_thread_num"
 },
 "unit":"number",
 "value":3
}

I will add a new JsonMetricVisitor to handle the convertion.
It's not to modify existing PrometheusMetricVisitor and SimpleCoreMetricVisitor.
Also I add a unit item to indicate the metric better.

public enum MetricUnit {
 NANOSECONDS,
 MICROSECONDS,
 MILLISECONDS,
 SECONDS,
 BYTES,
 ROWS,
 NUMBER,
 PERCENT,
 NOUNIT
};