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

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"`
}