Compute cpu instantaneous usage in the core

[vishh]

As of now cpu instantaneous usage metrics are computed on the fly, in the REST handler. Instead they should be computed in the core.

cc @timstclair @pwittrock

[mboussaa]

the only way I found to get the cpu usage per container is doing like following in Java:

 public static double getInstantPercentCPUUsage(JSONObject obj0) throws ParseException {

        JSONObject obj = obj0.getJSONObject(obj0.keys().next()) ;
        JSONObject firstValue = obj.getJSONArray("stats").getJSONObject(obj.getJSONArray("stats").length()-1);
        JSONObject secondValue = obj.getJSONArray("stats").getJSONObject(obj.getJSONArray("stats").length()-2);
        String ts1 = firstValue.getString("timestamp");
        String ts2 = secondValue.getString("timestamp");

        int cpuUsage1 = firstValue.getJSONObject("cpu").getJSONObject("usage").getInt("total");
        int cpuUsage2 = secondValue.getJSONObject("cpu").getJSONObject("usage").getInt("total");

        DateTimeFormatter parser = ISODateTimeFormat.dateTime();
        DateTime dt1 = parser.parseDateTime(ts1);
        DateTime dt2 = parser.parseDateTime(ts2);

        DateTimeFormatter formatter = DateTimeFormat.mediumDateTime();
        return (cpuUsage1-cpuUsage2)/((dt1.getSecondOfDay()-dt2.getSecondOfDay())*10000000) ;
    }

It is the derivative cpu usage over time. I don't know if that right !!