### Environment

* Dubbo version: 2.7.3
* Operating System version: linux
* Java version:1.8

### Steps to reproduce this issue
带"monitor"参数进行回声测试时抛返回异常
ause: org.apache.dubbo.remoting.RemotingException: java.util.concurrent.CompletionException: java.lang.NumberFormatException: null java.util.concurrent.CompletionException: java.lang.NumberFormatException: null at 

抛异常的位置在MonitorFilter的onResponse
![image](https://user-images.githubusercontent.com/7105269/85144468-c4c32180-b27d-11ea-8472-69aaab4ea72c.png)

大致原因：
EchoFilter在MonitorFilter之前执行并终止Invoker链，导致MonitorFilter没有执行，invocation没有设置key“monitor_filter_start_time” 
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package org.apache.dubbo.monitor.support;
import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.extension.Activate;
import org.apache.dubbo.common.logger.Logger;