private void checkAndDelNull(Map<String, String> metaData) {
        metaData.entrySet().removeIf((e) -> e.getValue() == null);
    }
the main logic of polaris registry, similar to consul

@author <a href=mailto:bner666@gmail.com>ZhangLibin</a>
registry/registry-polaris/src/main/java/com/alipay/sofa/rpc/registry/polaris/PolarisRegistry.java