+ /**
+ * Choose a thread for the next task.
+ *
+ * It will first check for and return an idle thread.
+ * If all threads are busy, then it will try to create a new one up to the `max` thread count.
+ * If the max thread count is reached, the emitter will emit a `FullPool` event and it will fall back to using a round robin algorithm to distribute the load.
+ *
+ * @returns Thread worker.
+ */