### 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” 
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("xxx", lastStatistics.getParameter(MonitorService.METHOD));
        Assertions.assertEquals(NetUtils.getLocalHost() + ":20880", lastStatistics.getParameter(MonitorService.PROVIDER));
        Assertions.assertEquals(NetUtils.getLocalHost(), lastStatistics.getAddress());
        Assertions.assertNull(lastStatistics.getParameter(MonitorService.CONSUMER));
        Assertions.assertEquals(1, lastStatistics.getParameter(MonitorService.SUCCESS, 0));
        Assertions.assertEquals(0, lastStatistics.getParameter(MonitorService.FAILURE, 0));
        Assertions.assertEquals(1, lastStatistics.getParameter(MonitorService.CONCURRENT, 0));
        Assertions.assertEquals(invocation, lastInvocation);
    }
    @Test
    public void testSafeFailForMonitorCollectFail() {
        MonitorFilter monitorFilter = new MonitorFilter();
        MonitorFactory mockMonitorFactory = mock(MonitorFactory.class);
        Monitor mockMonitor = mock(Monitor.class);
        Mockito.doThrow(new RuntimeException()).when(mockMonitor).collect(any(URL.class));
        monitorFilter.setMonitorFactory(mockMonitorFactory);
        given(mockMonitorFactory.getMonitor(any(URL.class))).willReturn(mockMonitor);
        Invocation invocation = new RpcInvocation("aaa", MonitorService.class.getName(), "", new Class<?>[0], new Object[0]);
        monitorFilter.invoke(serviceInvoker, invocation);
    }
}