@Override
    public T pop() {
        if (isOverflow() && overflowIndex > 0) {
            overflowIndex--;
            return factory.disableInstance();
        }

        final T spanEvent = peek();
        if (spanEvent != null) {
            stack[index - 1] = null;
            index--;
        }
        return spanEvent;
    }
@author netspider
@author Woonduk Kang(emeroad)
@author jaehong.kim
agent-module/profiler/src/main/java/com/navercorp/pinpoint/profiler/context/DefaultCallStack.java