Add task pending event

modules/nextflow/src/main/groovy/nextflow/Session.groovy

@@ -944,6 +944,18 @@ class Session implements ISession {
         }
     }
 
+    void notifyTaskPending( TaskHandler handler ) {
+        for( int i=0; i<observers.size(); i++ ) {
+            final observer = observers.get(i)
+            try {
+                observer.onProcessPending(handler, handler.getTraceRecord())
+            }
+            catch( Exception e ) {
+                log.debug(e.getMessage(), e)
+            }
+        }
+    }
+
     /**
      * Notifies that a task has been submitted
      */

modules/nextflow/src/main/groovy/nextflow/processor/TaskPollingMonitor.groovy

@@ -225,6 +225,7 @@ class TaskPollingMonitor implements TaskMonitor {
         try{
             pendingQueue << handler
             taskAvail.signal()  // signal that a new task is available for execution
+            session.notifyTaskPending(handler)
             log.trace "Scheduled task > $handler"
         }
         finally {

modules/nextflow/src/main/groovy/nextflow/trace/TraceObserver.groovy

@@ -48,6 +48,16 @@ trait TraceObserver {
       */
     void onProcessTerminate( TaskProcessor process ){}
 
+    /**
+     * This method when a new task is created and submitted in the nextflow
+     * internal queue of pending task to be scheduled to the underlying
+     * execution backend
+     *
+     * @param handler
+     * @param trace
+     */
+    void onProcessPending(TaskHandler handler, TraceRecord trace){}
+
     /**
      * This method is invoked before a process run is going to be submitted
      *