code: func getStringMultipleStrict(labels map[string]string, labelNames ...string) (map[string]string, error) {
  	foundLabels := map[string]string{}
  	for _, name := range labelNames {
  		value := getStringValue(labels, name, "")
  		// Error out only if one of them is not defined.
  		if len(value) == 0 {
  			return nil, fmt.Errorf("label not found: %s", name)
  		}
  		foundLabels[name] = value
  	}
  	return foundLabels, nil
  }
docs: // getStringMultipleStrict get multiple string values associated to several labels
  // Fail if one label is missing