454291 Added busy threads JMX attribute to QueuedThreadPool

Also-by: Tomasz Nurkiewicz <nurkiewicz@gmail.com>
Signed-off-by: Greg Wilkins <gregw@intalio.com>

jetty-util/src/main/java/org/eclipse/jetty/util/thread/QueuedThreadPool.java

@@ -405,6 +405,15 @@ public class QueuedThreadPool extends AbstractLifeCycle implements SizedThreadPo
         return _threadsIdle.get();
     }
 
+    /**
+     * @return The number of busy threads in the pool
+     */
+    @ManagedAttribute("total number of busy threads in the pool")
+    public int getBusyThreads()
+    {
+        return getThreads() - getIdleThreads();
+    }
+    
     /**
      * @return True if the pool is at maxThreads and there are not more idle threads than queued jobs
      */