### 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” 
return URL.valueOf("dubbo://" + NetUtils.getLocalHost() + ":20880?" +
                    APPLICATION_KEY + "=abc&" + SIDE_KEY + "=" + CONSUMER_SIDE)
                    .putAttribute(MONITOR_KEY, URL.valueOf("dubbo://" + NetUtils.getLocalHost() + ":7070"));
        }
        @Override
        public boolean isAvailable() {
            return false;
        }
        @Override
        public Result invoke(Invocation invocation) throws RpcException {
            lastInvocation = invocation;
            return AsyncRpcResult.newDefaultAsyncResult(invocation);
        }
        @Override
        public void destroy() {
        }
    };
    private MonitorFactory monitorFactory = new MonitorFactory() {
        @Override
        public Monitor getMonitor(final URL url) {
            return new Monitor() {
                public URL getUrl() {
                    return url;
                }
                @Override
                public boolean isAvailable() {
                    return true;
                }
                @Override
                public void destroy() {