private String buildExtJson(final UpstreamInstance instance, final DocItem docItem) {
        ApiDocRegisterDTO.ApiExt ext = new ApiDocRegisterDTO.ApiExt();
        ext.setHost(instance.getIp());
        ext.setPort(instance.getPort());
        ext.setServiceName(instance.getClusterName());
        ext.setMethodName(docItem.getName());
        ext.setParameterTypes("");
        ext.setRpcExt(null);
        ext.setAddPrefixed(false);
        ext.setProtocol(HTTP);
        return GsonUtils.getInstance().toJson(ext);
    }
add docInfo.

@param instance    instance
@param docInfoJson docInfoJson
@param callback    callback
shenyu-admin/src/main/java/org/apache/shenyu/admin/service/manager/impl/DocManagerImpl.java