[WIP] Add a metric to show the average number of free workers available.

source/taskmanager/src/main/java/nl/aerius/taskmanager/QueueWatchDog.java

@@ -69,7 +69,7 @@ public void onWorkerFinished(final String messageId, final Map<String, Object> m
   }
 
   @Override
-  public void onNumberOfWorkersUpdate(final int numberOfWorkers, final int numberOfMessages) {
+  public void onNumberOfWorkersUpdate(final int numberOfWorkers, final int numberOfMessages, int numberOfMessagesInProgress) {
     if (isItDead(!runningTasks.isEmpty(), numberOfMessages)) {
       LOG.info("It looks like some tasks are zombies on {} worker queue, so all tasks currently in state running are released.", workerQueueName);
       listeners.forEach(QueueWatchDogListener::reset);

source/taskmanager/src/main/java/nl/aerius/taskmanager/StartupGuard.java

@@ -22,8 +22,8 @@
 
 /**
  * Class to be used at startup. The Scheduler should not start before it is known how many messages are still on the queue.
- * This to register any work that is still on the queuue and to properly calculate load metrics.
- * Because the Taskmanager is not aware of the tasks already on the queue and therefore otherwise these messaages won't be counted in the metrics.
+ * This to register any work that is still on the queue and to properly calculate load metrics.
+ * Because the Task Manager is not aware of the tasks already on the queue and therefore otherwise these messages won't be counted in the metrics.
  * This can result in the metrics being skewed, and thereby negatively reporting load metrics.
  */
 public class StartupGuard implements WorkerSizeObserver {
@@ -47,7 +47,7 @@ public void waitForOpen() throws InterruptedException {
   }
 
   @Override
-  public void onNumberOfWorkersUpdate(final int numberOfWorkers, final int numberOfMessages) {
+  public void onNumberOfWorkersUpdate(final int numberOfWorkers, final int numberOfMessages, final int numberOfMessagesInProgress) {
     synchronized (openSemaphore) {
       if (!open) {
         open = true;