code: func (t *QueueManager) Append(s []tsdb.RefSample) bool {
  	type enqueuable struct {
  		ts  prompb.TimeSeries
  		ref uint64
  	}
  
  	tempSamples := make([]enqueuable, 0, len(s))
  	t.seriesMtx.Lock()
  	for _, sample := range s {
  		// If we have no labels for the series, due to relabelling or otherwise, don't send the sample.
  		if _, ok := t.seriesLabels[sample.Ref]; !ok {
  			t.droppedSamplesTotal.Inc()
  			t.samplesDropped.incr(1)
  			if _, ok := t.droppedSeries[sample.Ref]; !ok {
  				level.Info(t.logger).Log("msg", "dropped sample for series that was not explicitly dropped via relabelling", "ref", sample.Ref)
  			}
  			continue
  		}
  		tempSamples = append(tempSamples, enqueuable{
  			ts: prompb.TimeSeries{
  				Labels: t.seriesLabels[sample.Ref],
  				Samples: []prompb.Sample{
  					prompb.Sample{
  						Value:     float64(sample.V),
  						Timestamp: sample.T,
  					},
  				},
  			},
  			ref: sample.Ref,
  		})
  	}
  	t.seriesMtx.Unlock()
  
  outer:
  	for _, sample := range tempSamples {
  		// This will only loop if the queues are being resharded.
  		backoff := t.cfg.MinBackoff
  		for {
  			select {
  			case <-t.quit:
  				return false
  			default:
  			}
  
  			if t.shards.enqueue(sample.ref, sample.ts) {
  				continue outer
  			}
  
  			t.enqueueRetriesMetric.Inc()
  			time.Sleep(time.Duration(backoff))
  			backoff = backoff * 2
  			if backoff > t.cfg.MaxBackoff {
  				backoff = t.cfg.MaxBackoff
  			}
  		}
  	}
  	return true
  }
docs: // Append queues a sample to be sent to the remote storage. Blocks until all samples are
  // enqueued on their shards or a shutdown signal is received.