Fix leaking go routine in docker stats fetching

[ruflin]

See #3489

[andrewkroh]

Consider swapping things around so that you can call wg.Done() from the worker goroutines. This more closely aligns with the Fan-in, Fan-out examples here.

 	for _, container := range containers {
 		go func(container docker.APIContainers) {
			defer wg.Done()
 			queue <- exportContainerStats(client, &container)
 		}(container)
 	}

 	go func() {
  	  	wg.Wait()
  	  	close(queue)
  	}()
  
  	// This will break after the queue has been drained and queue is closed.
 	for container := range queue {
 		containersList = append(containersList, container)
 	}

[andrewkroh]

And up on line 69, consider changing it to containersList = make([]Stat, 0, len(containers)) since we know ahead of time how much capacity the slice needs. This will reduce the number of allocations.

[ruflin]

This definitively makes the code much cleaner.