code: func (f *asyncFlusher) sendChunk(c chunk) {
  	c.index = atomic.AddInt32(&f.index, 1)
  	logging.Infof(f.ctx(c.index), "Chunk with %d batches queued", len(c.Tasks))
  	f.chunks <- c
  }
docs: // sendChunk starts an asynchronous operation to upload data to BigQuery.
  //
  // Can block if too many parallel uploads are already underway. Panics if called
  // before 'start' or after 'waitAll'.
  //
  // On successful upload it deletes the tasks from Pull Queue.