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
};