apollo-openapi/src/main/java/com/ctrip/framework/apollo/openapi/client/ApolloOpenApiClient.java/ApolloOpenApiClient/getItem
class ApolloOpenApiClient:  public OpenItemDTO getItem(String appId, String env, String clusterName, String namespaceName, String key) {
    return itemService.getItem(appId, env, clusterName, namespaceName, key);
  }