### 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” 
}
                public void collect(URL statistics) {
                    MonitorFilterTest.this.lastStatistics = statistics;
                }
                public List<URL> lookup(URL query) {
                    return Arrays.asList(MonitorFilterTest.this.lastStatistics);
                }
            };
        }
    };
    @Test
    public void testFilter() throws Exception {
        MonitorFilter monitorFilter = new MonitorFilter();
        monitorFilter.setMonitorFactory(monitorFactory);
        Invocation invocation = new RpcInvocation("aaa", MonitorService.class.getName(), "", new Class<?>[0], new Object[0]);
        RpcContext.getServiceContext().setRemoteAddress(NetUtils.getLocalHost(), 20880).setLocalAddress(NetUtils.getLocalHost(), 2345);
        Result result = monitorFilter.invoke(serviceInvoker, invocation);
        result.whenCompleteWithContext((r, t) -> {
            if (t == null) {
                monitorFilter.onResponse(r, serviceInvoker, invocation);
            } else {
                monitorFilter.onError(t, serviceInvoker, invocation);
            }
        });
        while (lastStatistics == null) {
            Thread.sleep(10);
        }
        Assertions.assertEquals("abc", lastStatistics.getParameter(MonitorService.APPLICATION));
        Assertions.assertEquals(MonitorService.class.getName(), lastStatistics.getParameter(MonitorService.INTERFACE));
        Assertions.assertEquals("aaa", lastStatistics.getParameter(MonitorService.METHOD));