code: func NewConstMetric(desc *Desc, valueType ValueType, value float64, labelValues ...string) (Metric, error) {
  	if desc.err != nil {
  		return nil, desc.err
  	}
  	if err := validateLabelValues(labelValues, len(desc.variableLabels)); err != nil {
  		return nil, err
  	}
  	return &constMetric{
  		desc:       desc,
  		valType:    valueType,
  		val:        value,
  		labelPairs: makeLabelPairs(desc, labelValues),
  	}, nil
  }
docs: // NewConstMetric returns a metric with one fixed value that cannot be
  // changed. Users of this package will not have much use for it in regular
  // operations. However, when implementing custom Collectors, it is useful as a
  // throw-away metric that is generated on the fly to send it to Prometheus in
  // the Collect method. NewConstMetric returns an error if the length of
  // labelValues is not consistent with the variable labels in Desc or if Desc is
  // invalid.