itemsBeingProcessed is always 1 regardless of worker count

[jfernandez]

At Netflix, we are running a custom fork with this change to expose the itemsBeingProcessed map size: #1162

We have implemented a gauge metric to monitor the size of itemsBeingProcessed at short intervals. Despite having a significant queue size and operating with 20 workers, the maximum value observed for this metric is 1, indicating only one item is being processed at a time, contrary to our expectation of parallel processing by multiple workers.

We suspect that the waitForNextItemSemaphore might be the cause of this serialization. Is this a possible explanation?