func (s *CustomAttributesService) ListCustomProjectAttributes(project int, options ...OptionFunc) ([]*CustomAttribute, *Response, error) {
	return s.listCustomAttributes("projects", project, options...)
}
ListCustomProjectAttributes lists the custom attributes of the specified project.

GitLab API docs:
https://docs.gitlab.com/ce/api/custom_attributes.html#list-custom-attributes
package gitlab
import (
	"fmt"
)
type CustomAttributesService struct {
	client *Client
}
type CustomAttribute struct {
	Key   string `json:"key"`
	Value string `json:"value"`
}
func (s *CustomAttributesService) ListCustomUserAttributes(user int, options ...OptionFunc) ([]*CustomAttribute, *Response, error) {
	return s.listCustomAttributes("users", user, options...)
}
func (s *CustomAttributesService) ListCustomGroupAttributes(group int, options ...OptionFunc) ([]*CustomAttribute, *Response, error) {
	return s.listCustomAttributes("groups", group, options...)
}