### 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” 
int concurrent = getConcurrent(invoker, invocation).get(); 
        String application = invoker.getUrl().getApplication();
        String service = invoker.getInterface().getName(); 
        String method = RpcUtils.getMethodName(invocation); 
        String group = invoker.getUrl().getGroup();
        String version = invoker.getUrl().getVersion();
        int localPort;
        String remoteKey, remoteValue;
        if (CONSUMER_SIDE.equals(invoker.getUrl().getSide())) {
            
            localPort = 0;
            remoteKey = MonitorService.PROVIDER;
            remoteValue = invoker.getUrl().getAddress();
        } else {
            
            localPort = invoker.getUrl().getPort();
            remoteKey = MonitorService.CONSUMER;
            remoteValue = remoteHost;
        }
        String input = "", output = "";
        if (invocation.getAttachment(INPUT_KEY) != null) {
            input = invocation.getAttachment(INPUT_KEY);
        }
        if (result != null && result.getAttachment(OUTPUT_KEY) != null) {
            output = result.getAttachment(OUTPUT_KEY);
        }
        return new ServiceConfigURL(COUNT_PROTOCOL, NetUtils.getLocalHost(), localPort, service + PATH_SEPARATOR + method, MonitorService.APPLICATION, application, MonitorService.INTERFACE, service, MonitorService.METHOD, method, remoteKey, remoteValue, error ? MonitorService.FAILURE : MonitorService.SUCCESS, "1", MonitorService.ELAPSED, String.valueOf(elapsed), MonitorService.CONCURRENT, String.valueOf(concurrent), INPUT_KEY, input, OUTPUT_KEY, output, GROUP_KEY, group, VERSION_KEY, version);
    }
}