title: Autoscale stuck at 1, never see getSplitBacklogBytes() execute
body: <p>I am creating a custom IO connector in Java that runs is streaming mode with 2.8.0 SDK.  It reads from a messaging queue for further pipeline processing, similar to JMS IO.  All works well except the auto-scaling does not seem to work.  The odd part for me is that I have implemented a getSplitBacklogBytes() method, but as far as the logs show, it is never executed.</p>
  
  <p>I am testing with a modified wordcount example to test, executed with:</p>
  
  <pre><code>--runner=DataflowRunner --autoscalingAlgorithm=THROUGHPUT_BASED  --maxNumWorkers=4
  </code></pre>
  
  <p>The Pipeline options and logs seem to show that auto-scaling is being set up correctly.</p>
  
  <pre><code>PipelineOptions: Current Settings:
    autoscalingAlgorithm: THROUGHPUT_BASED
    maxNumWorkers: 4
    numWorkers: 0
  </code></pre>
  
  <p>Logs show:</p>
  
  <pre><code>20:50:54:977 .. [INFO] .. - 2019-01-08T20:50:40.937Z: Autoscaling is enabled for job 2019-01-08_12_50_40-3227727519433807139. The number of workers will be between 1 and 4.
  20:50:54:991 .. [INFO] .. - 2019-01-08T20:50:42.943Z: Autoscaling: Enabled for job 2019-01-08_12_50_40-3227727519433807139 between 1 and 4 worker processes.
  20:50:55:052 .. [INFO] .. - 2019-01-08T20:50:49.186Z: Generating 4 persistent disks which will allow autoscaling from 1 to 4 workers.
  20:50:55:053 .. [INFO] .. - 2019-01-08T20:50:49.493Z: Starting 1 workers...
  </code></pre>
  
  <p>Is there anything else that needs to be done to enable this feature, or anything I might have done to inadvertently disable this feature?</p>
  
  <p>Source code lives here:
  <a href="https://github.com/SolaceLabs/solace-beam-integration/tree/development" rel="nofollow noreferrer">https://github.com/SolaceLabs/solace-beam-integration/tree/development</a></p>