feat(tokio-util): EventListeners with Listener Management (#5136)

Co-authored-by: Matthias Seitz <matthias.seitz@outlook.de>

crates/tokio-util/src/event_listeners.rs

@@ -33,4 +33,14 @@ impl<T: Clone> EventListeners<T> {
     pub fn push_listener(&mut self, listener: mpsc::UnboundedSender<T>) {
         self.listeners.push(listener);
     }
+
+    /// Returns the number of registered listeners.
+    pub fn len(&self) -> usize {
+        self.listeners.len()
+    }
+
+    /// Returns true if there are no registered listeners.
+    pub fn is_empty(&self) -> bool {
+        self.listeners.is_empty()
+    }
 }