### 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” 
private static final Logger logger = LoggerFactory.getLogger(MonitorFilter.class);
    private static final String MONITOR_FILTER_START_TIME = "monitor_filter_start_time";
    private static final String MONITOR_REMOTE_HOST_STORE = "monitor_remote_host_store";
    /**
     * The Concurrent counter
     */
    private final ConcurrentMap<String, AtomicInteger> concurrents = new ConcurrentHashMap<String, AtomicInteger>();
    /**
     * The MonitorFactory
     */
    private MonitorFactory monitorFactory;
    public void setMonitorFactory(MonitorFactory monitorFactory) {
        this.monitorFactory = monitorFactory;
    }
    /**
     * The invocation interceptor,it will collect the invoke data about this invocation and send it to monitor center
     *
     * @param invoker    service
     * @param invocation invocation.
     * @return {@link Result} the invoke result
     * @throws RpcException
     */
    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        if (invoker.getUrl().hasAttribute(MONITOR_KEY)) {
            invocation.put(MONITOR_FILTER_START_TIME, System.currentTimeMillis());
            invocation.put(MONITOR_REMOTE_HOST_STORE, RpcContext.getServiceContext().getRemoteHost());
            getConcurrent(invoker, invocation).incrementAndGet(); 
        }
        return invoker.invoke(invocation);