public String getItemValue(final String key) {
        OpenItemDTO openItemDTO = this.apolloOpenApiClient.getItem(
                apolloConfig.getAppId(),
                apolloConfig.getEnv(),
                apolloConfig.getClusterName(),
                apolloConfig.getNamespace(),
                key
        );
        return openItemDTO.getValue();
    }
get item value.
@param key item key
@return item value
shenyu-registry/shenyu-registry-apollo/src/main/java/org/apache/shenyu/registry/apollo/ApolloClient.java