### Environment

* Dubbo version: 2.7.5
* Operating System version: win10 or centos7
* Java version: 1.8.0
* Zookeeper version: 3.5.6

### Steps to reproduce this issue
My previous dubbo service version was 2.7.3. Today, I upgraded to 2.7.5. I found that the metadata can't be displayed all the time. The dubbo service registry in zookeeper, but zookeeper not /dubbo/metadata node path view.  If I switch back to 2.7.3 and 2.7.4.1, it can be displayed. Are there any bugs? I need help~~

xml-config:
`<dubbo:metadata-report address="zookeeper://192.168.11.100:2181?backup=192.168.11.105:2181" />`
```

String CLASSIFIER_KEY = "classifier";
    String VERSION_KEY = "version";
    String REVISION_KEY = "revision";
    String METADATA_REVISION = "metadata.revision";
    String METADATA_KEY = "metadata";
    String DEFAULT_METADATA_STORAGE_TYPE = "local";
    String REMOTE_METADATA_STORAGE_TYPE = "remote";
    /**
     * Consumer side 's proxy class
     */
    String PROXY_CLASS_REF = "refClass";
    /**
     * generic call
     */
    String $INVOKE = "$invoke";
    String $INVOKE_ASYNC = "$invokeAsync";
    /**
     * package version in the manifest
     */
    String RELEASE_KEY = "release";
    int MAX_PROXY_COUNT = 65535;
    String MONITOR_KEY = "monitor";
    String CLUSTER_KEY = "cluster";
    String USERNAME_KEY = "username";
    String PASSWORD_KEY = "password";
    String HOST_KEY = "host";
    String PORT_KEY = "port";
    String DUBBO_IP_TO_BIND = "DUBBO_IP_TO_BIND";
    @Deprecated
    String SHUTDOWN_WAIT_SECONDS_KEY = "dubbo.service.shutdown.wait.seconds";