check.go
func (c *Check) AddPerfDatum(label, unit string, value float64, thresholds ...float64) error {
	datum, err := NewPerfDatum(label, unit, value, thresholds...)
	if err != nil {
		return err
	}
	c.perfdata = append(c.perfdata, *datum)
	return nil
}
func (c *Check) AddPerfDatum(label, unit string, value float64, thresholds ...float64) error {
	datum, err := NewPerfDatum(label, unit, value, thresholds...)
	if err != nil {
		return err
	}
	c.perfdata = append(c.perfdata, *datum)
	return nil
}
// AddPerfDatum adds a metric to the set output by the check. unit must
// be a valid Nagios unit of measurement (UOM): "us", "ms", "s",
// "%", "b", "kb", "mb", "gb", "tb", "c", or the empty string. UOMs are
// not case-sensitive.
//
// Zero or more of the thresholds min, max, warn and crit may be
// supplied; these must be of the same UOM as the value.
//
// A threshold may be positive or negative infinity, in which case it
// will be omitted in the check output. A value may not be either
// infinity.
//
// Returns error on invalid parameters.