Expose ThreadPoolTaskExecutor queue size and capacity for metrics

We use Grafana to monitor our app via Spring's JMX exporter, and we
think it could be interesting to have at least the current queue size
for this purpose since the queue size directly affects the app memory
load. Having the queue capacity seems also interesting to set up
triggers whose values are calculated based on the maximum capacity of
the queue.

This commit introduces new getCurrentQueueSize() and getQueueCapacity()
methods in ThreadPoolTaskExecutor.

See gh-28583

spring-context/src/main/java/org/springframework/scheduling/concurrent/ThreadPoolTaskExecutor.java

@@ -315,6 +315,20 @@ public int getPoolSize() {
 		}
 		return this.threadPoolExecutor.getPoolSize();
 	}
+
+	/**
+	 * Return the current number of threads waiting in the queue
+	 */
+	public int getCurrentQueueSize() {
+		return this.getThreadPoolExecutor().getQueue().size();
+	}
+
+	/**
+	* Return the maximum capacity of the queue
+	*/
+	public int getQueueCapacity() {
+		return this.queueCapacity;
+	}
 
 	/**
 	 * Return the number of currently active threads.