Add a warning about SingleThreadEventExecutor.pendingTasks() operation

common/src/main/java/io/netty/util/concurrent/SingleThreadEventExecutor.java

@@ -262,6 +262,9 @@ public abstract class SingleThreadEventExecutor extends AbstractEventExecutor {
 
     /**
      * Return the number of tasks that are pending for processing.
+     *
+     * <strong>Be aware that this operation may be expensive as it depends on the internal implementation of the
+     * SingleThreadEventExecutor. So use it was care!</strong>
      */
     public final int pendingTasks() {
         return taskQueue.size();