The update_frame_count system should be placed in CorePlugin (#6676)

# Objective

Latest Release, "bevy 0.9" move the FrameCount updater into RenderPlugin, it leads to user who only run app with Core/Minimal Plugin cannot get the right number of FrameCount, it always return 0.

As for use cases like a server app, we don't want to add render dependencies to the app.

More detail in #6656 

## Solution

- Move the `update_frame_count` into CorePlugin

crates/bevy_core/src/lib.rs

@@ -6,7 +6,7 @@ mod name;
 mod serde;
 mod task_pool_options;
 
-use bevy_ecs::system::Resource;
+use bevy_ecs::system::{ResMut, Resource};
 pub use bytemuck::{bytes_of, cast_slice, Pod, Zeroable};
 pub use name::*;
 pub use task_pool_options::*;
@@ -53,6 +53,7 @@ impl Plugin for CorePlugin {
         register_math_types(app);
 
         app.init_resource::<FrameCount>();
+        app.add_system(update_frame_count);
     }
 }
 
@@ -108,6 +109,10 @@ fn register_math_types(app: &mut App) {
 #[derive(Default, Resource, Clone, Copy)]
 pub struct FrameCount(pub u32);
 
+fn update_frame_count(mut frame_count: ResMut<FrameCount>) {
+    frame_count.0 = frame_count.0.wrapping_add(1);
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -145,4 +150,14 @@ mod tests {
         compute_rx.try_recv().unwrap();
         io_rx.try_recv().unwrap();
     }
+
+    #[test]
+    fn frame_counter_update() {
+        let mut app = App::new();
+        app.add_plugin(CorePlugin::default());
+        app.update();
+
+        let frame_count = app.world.resource::<FrameCount>();
+        assert_eq!(1, frame_count.0);
+    }
 }

crates/bevy_render/src/globals.rs

@@ -18,12 +18,17 @@ impl Plugin for GlobalsPlugin {
             render_app
                 .init_resource::<GlobalsBuffer>()
                 .init_resource::<Time>()
+                .add_system_to_stage(RenderStage::Extract, extract_frame_count)
                 .add_system_to_stage(RenderStage::Extract, extract_time)
                 .add_system_to_stage(RenderStage::Prepare, prepare_globals_buffer);
         }
     }
 }
 
+fn extract_frame_count(mut commands: Commands, frame_count: Extract<Res<FrameCount>>) {
+    commands.insert_resource(**frame_count);
+}
+
 fn extract_time(mut commands: Commands, time: Extract<Res<Time>>) {
     commands.insert_resource(time.clone());
 }

crates/bevy_render/src/lib.rs

@@ -19,7 +19,6 @@ mod spatial_bundle;
 pub mod texture;
 pub mod view;
 
-use bevy_core::FrameCount;
 use bevy_hierarchy::ValidParentCheckPlugin;
 pub use extract_param::Extract;
 
@@ -333,8 +332,7 @@ impl Plugin for RenderPlugin {
             .add_plugin(CameraPlugin)
             .add_plugin(ViewPlugin)
             .add_plugin(MeshPlugin)
-            .add_plugin(GlobalsPlugin)
-            .add_plugin(FrameCountPlugin);
+            .add_plugin(GlobalsPlugin);
     }
 }
 
@@ -368,22 +366,3 @@ fn extract(app_world: &mut World, render_app: &mut App) {
     // see <https://github.com/bevyengine/bevy/issues/5082>
     extract.apply_buffers(running_world);
 }
-
-pub struct FrameCountPlugin;
-impl Plugin for FrameCountPlugin {
-    fn build(&self, app: &mut bevy_app::App) {
-        app.add_system(update_frame_count);
-
-        if let Ok(render_app) = app.get_sub_app_mut(RenderApp) {
-            render_app.add_system_to_stage(RenderStage::Extract, extract_frame_count);
-        }
-    }
-}
-
-fn update_frame_count(mut frame_count: ResMut<FrameCount>) {
-    frame_count.0 = frame_count.0.wrapping_add(1);
-}
-
-fn extract_frame_count(mut commands: Commands, frame_count: Extract<Res<FrameCount>>) {
-    commands.insert_resource(**frame_count);
-}