code: func (p *ProductBuilder) Build() {
  	elapsed := rand.Intn(p.bound)
  
  	time.Sleep(time.Duration(elapsed) * time.Millisecond)
  
  	p.totalTime.MustInc(float64(elapsed))
  	p.completed.Up()
  }
docs: // Build sleeps for a random time, then adds that value to totalTime