code: func (g *PCPGaugeVector) Val(instance string) (float64, error) {
  	g.mutex.RLock()
  	defer g.mutex.RUnlock()
  
  	val, err := g.valInstance(instance)
  	if err != nil {
  		return 0, err
  	}
  
  	return val.(float64), nil
  }
docs: // Val returns the value of a particular instance of PCPGaugeVector