code: func (g *PCPGaugeVector) Inc(inc float64, instance string) error {
  	g.mutex.Lock()
  	defer g.mutex.Unlock()
  
  	v, err := g.valInstance(instance)
  	if err != nil {
  		return err
  	}
  
  	return g.setInstance(v.(float64)+inc, instance)
  }
docs: // Inc increments the value of a particular instance of PCPGaugeVector