AsyncPool: expose workerUsage to the API

this allows plugins (and maybe later on the core) to detect async worker overload and warn the user about potential performance issues.
I planned to implement such detection in the core directly, but it turned out to be a bit more complex than I anticipated. At the least, this API might be useful to someone else.

src/pocketmine/scheduler/AsyncPool.php

@@ -329,6 +329,16 @@ class AsyncPool{
 		$this->collectWorkers();
 	}
 
+	/**
+	 * Returns an array of worker ID => task queue size
+	 *
+	 * @return int[]
+	 * @phpstan-return array<int, int>
+	 */
+	public function getTaskQueueSizes() : array{
+		return $this->workerUsage;
+	}
+
 	public function shutdownUnusedWorkers() : int{
 		$ret = 0;
 		$time = time();