code: @Override
  	public long getCount(long when){
  		Long recWhen = when - (when % granularity);
  		AtomicLong times = counters.get(recWhen);
  		return times == null ? 0 : times.longValue(); 
  	}
docs: 获取在某时刻的计数值。
  @param when	时刻
  @return		统计数