code: func NewPCPInstanceMetric(vals Instances, name string, indom *PCPInstanceDomain, t MetricType, s MetricSemantics, u MetricUnit, desc ...string) (*PCPInstanceMetric, error) {
  	d, err := newpcpMetricDesc(name, t, s, u, desc...)
  	if err != nil {
  		return nil, err
  	}
  
  	im, err := newpcpInstanceMetric(vals, indom, d)
  	if err != nil {
  		return nil, err
  	}
  
  	return &PCPInstanceMetric{im, sync.RWMutex{}}, nil
  }
docs: // NewPCPInstanceMetric creates a new instance of PCPSingletonMetric.
  // it takes 2 extra optional strings as short and long description parameters,
  // which on not being present are set to empty strings.