### 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” 
import org.apache.dubbo.common.logger.LoggerFactory;
import org.apache.dubbo.common.url.component.ServiceConfigURL;
import org.apache.dubbo.common.utils.NetUtils;
import org.apache.dubbo.monitor.Monitor;
import org.apache.dubbo.monitor.MonitorFactory;
import org.apache.dubbo.monitor.MonitorService;
import org.apache.dubbo.rpc.Filter;
import org.apache.dubbo.rpc.Invocation;
import org.apache.dubbo.rpc.Invoker;
import org.apache.dubbo.rpc.Result;
import org.apache.dubbo.rpc.RpcContext;
import org.apache.dubbo.rpc.RpcException;
import org.apache.dubbo.rpc.support.RpcUtils;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicInteger;
import static org.apache.dubbo.common.constants.CommonConstants.CONSUMER_SIDE;
import static org.apache.dubbo.common.constants.CommonConstants.GROUP_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.MONITOR_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.PATH_SEPARATOR;
import static org.apache.dubbo.common.constants.CommonConstants.PROVIDER;
import static org.apache.dubbo.common.constants.CommonConstants.VERSION_KEY;
import static org.apache.dubbo.monitor.Constants.COUNT_PROTOCOL;
import static org.apache.dubbo.rpc.Constants.INPUT_KEY;
import static org.apache.dubbo.rpc.Constants.OUTPUT_KEY;
/**
 * MonitorFilter. (SPI, Singleton, ThreadSafe)
 */
@Activate(group = {PROVIDER})
public class MonitorFilter implements Filter, Filter.Listener {