code: func (p *Backoff) GC() {
  	p.Lock()
  	defer p.Unlock()
  	now := p.Clock.Now()
  	for id, entry := range p.perItemBackoff {
  		if now.Sub(entry.lastUpdate) > p.maxDuration*2 {
  			// GC when entry has not been updated for 2*maxDuration
  			delete(p.perItemBackoff, id)
  		}
  	}
  }
docs: // Garbage collect records that have aged past maxDuration. Backoff users are expected
  // to invoke this periodically.