func (s *CustomAttributesService) GetCustomUserAttribute(user int, key string, options ...RequestOptionFunc) (*CustomAttribute, *Response, error) {
	return s.getCustomAttribute("users", user, key, options...)
}
GetCustomUserAttribute returns the user attribute with a speciifc key.

GitLab API docs:
https://docs.gitlab.com/ce/api/custom_attributes.html#single-custom-attribute
package gitlab
import (
	"fmt"
	"net/http"
)
type CustomAttributesService struct {
	client *Client
}
type CustomAttribute struct {
	Key   string `json:"key"`
	Value string `json:"value"`
}
func (s *CustomAttributesService) ListCustomUserAttributes(user int, options ...RequestOptionFunc) ([]*CustomAttribute, *Response, error) {
	return s.listCustomAttributes("users", user, options...)
}
func (s *CustomAttributesService) ListCustomGroupAttributes(group int, options ...RequestOptionFunc) ([]*CustomAttribute, *Response, error) {
	return s.listCustomAttributes("groups", group, options...)
}
func (s *CustomAttributesService) ListCustomProjectAttributes(project int, options ...RequestOptionFunc) ([]*CustomAttribute, *Response, error) {
	return s.listCustomAttributes("projects", project, options...)
}
func (s *CustomAttributesService) listCustomAttributes(resource string, id int, options ...RequestOptionFunc) ([]*CustomAttribute, *Response, error) {
	u := fmt.Sprintf("%s/%d/custom_attributes", resource, id)
	req, err := s.client.NewRequest(http.MethodGet, u, nil, options)
	if err != nil {
		return nil, nil, err
	}
	var cas []*CustomAttribute
	resp, err := s.client.Do(req, &cas)
	if err != nil {
		return nil, resp, err
	}
	return cas, resp, err
}