### 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" />`
```

Pattern COMMA_SPLIT_PATTERN = Pattern.compile("\\s*[,]+\\s*");
    String PATH_SEPARATOR = "/";
    String PROTOCOL_SEPARATOR = "://";
    String REGISTRY_SEPARATOR = "|";
    Pattern REGISTRY_SPLIT_PATTERN = Pattern.compile("\\s*[|;]+\\s*");
    Pattern D_REGISTRY_SPLIT_PATTERN = Pattern.compile("\\s*[|]+\\s*");
    String SEMICOLON_SEPARATOR = ";";
    Pattern SEMICOLON_SPLIT_PATTERN = Pattern.compile("\\s*[;]+\\s*");
    Pattern EQUAL_SPLIT_PATTERN = Pattern.compile("\\s*[=]+\\s*");
    String DEFAULT_PROXY = "javassist";
    String DEFAULT_DIRECTORY = "dubbo";
    String PROTOCOL_KEY = "protocol";
    String DEFAULT_PROTOCOL = "dubbo";
    String DEFAULT_THREAD_NAME = "Dubbo";
    int DEFAULT_CORE_THREADS = 0;
    int DEFAULT_THREADS = 200;
    String EXECUTOR_SERVICE_COMPONENT_KEY = ExecutorService.class.getName();
    String THREADPOOL_KEY = "threadpool";
    String THREAD_NAME_KEY = "threadname";
    String CORE_THREADS_KEY = "corethreads";
    String THREADS_KEY = "threads";
    String QUEUES_KEY = "queues";
    String ALIVE_KEY = "alive";
    String DEFAULT_THREADPOOL = "limited";
    String DEFAULT_CLIENT_THREADPOOL = "cached";
    String IO_THREADS_KEY = "iothreads";
    int DEFAULT_QUEUES = 0;
    int DEFAULT_ALIVE = 60 * 1000;
    String TIMEOUT_KEY = "timeout";
    int DEFAULT_TIMEOUT = 1000;