code: func (g *PCPGauge) Inc(val float64) error {
  	g.mutex.Lock()
  	defer g.mutex.Unlock()
  
  	if val == 0 {
  		return nil
  	}
  
  	v := g.val.(float64)
  	return g.set(v + val)
  }
docs: // Inc adds a value to the existing Gauge value.