feat(grpc): add exit_idle to LbPolicy trait (#2332)

Author: cjqzhao

grpc/src/client/load_balancing/child_manager.rs

@@ -262,6 +262,10 @@ impl<T: PartialEq + Hash + Eq + Send + Sync + 'static> LbPolicy for ChildManager
             self.resolve_child_controller(channel_controller, child_idx);
         }
     }
+
+    fn exit_idle(&mut self, _channel_controller: &mut dyn ChannelController) {
+        todo!("implement exit_idle")
+    }
 }
 
 struct WrappedController<'a> {

grpc/src/client/load_balancing/mod.rs

@@ -169,6 +169,10 @@ pub trait LbPolicy: Send {
     /// Called by the channel in response to a call from the LB policy to the
     /// WorkScheduler's request_work method.
     fn work(&mut self, channel_controller: &mut dyn ChannelController);
+
+    /// Called by the channel when an LbPolicy goes idle and the channel
+    /// wants it to start connecting to subchannels again.
+    fn exit_idle(&mut self, channel_controller: &mut dyn ChannelController);
 }
 
 /// Controls channel behaviors.

grpc/src/client/load_balancing/pick_first.rs

@@ -99,6 +99,10 @@ impl LbPolicy for PickFirstPolicy {
     }
 
     fn work(&mut self, channel_controller: &mut dyn ChannelController) {}
+
+    fn exit_idle(&mut self, _channel_controller: &mut dyn ChannelController) {
+        todo!("implement exit_idle")
+    }
 }
 
 struct OneSubchannelPicker {