Allow users to customize history length in FrameTimeDiagnosticsPlugin (#17259)

# Objective

I have an application where I'd like to measure average frame rate over
the entire life of the application, and it would be handy if I could
just configure this on the existing `FrameTimeDiagnosticsPlugin`.

Probably fixes #10948?

## Solution

Add `max_history_length` to `FrameTimeDiagnosticsPlugin`, and because
`smoothing_factor` seems to be based on history length, add that too.

## Discussion

I'm not totally sure that `DEFAULT_MAX_HISTORY_LENGTH` is a great
default for `FrameTimeDiagnosticsPlugin` (or any diagnostic?). That's
1/3 of a second at typical game frame rates. Moreover, the default print
interval for `LogDiagnosticsPlugin` is 1 second. So when the two are
combined, you are printing the average over the last third of the
duration between now and the previous print, which seems a bit wonky.
(related: #11429)

I'm pretty sure this default value discussed and the current value
wasn't totally arbitrary though.

Maybe it would be nice for `Diagnostic` to have a
`with_max_history_length_and_also_calculate_a_good_default_smoothing_factor`
method? And then make an explicit smoothing factor in
`FrameTimeDiagnosticsPlugin` optional?

Or add a `new(max_history_length: usize)` method to
`FrameTimeDiagnosticsPlugin` that sets a reasonable default
`smoothing_factor`? edit: This one seems like a no-brainer, doing it.

## Alternatives

It's really easy to roll your own `FrameTimeDiagnosticsPlugin`, but that
might not be super interoperable with, for example, third party FPS
overlays. Still, might be the right call.

## Testing

`cargo run --example many_sprites` (modified to use a custom
`max_history_length`)

## Migration Guide

`FrameTimeDiagnosticsPlugin` now contains two fields. Use
`FrameTimeDiagnosticsPlugin::default()` to match Bevy's previous
behavior or, for example, `FrameTimeDiagnosticsPlugin::new(60)` to
configure it.

Author: rparrett

crates/bevy_dev_tools/src/fps_overlay.rs

@@ -42,7 +42,7 @@ impl Plugin for FpsOverlayPlugin {
     fn build(&self, app: &mut bevy_app::App) {
         // TODO: Use plugin dependencies, see https://github.com/bevyengine/bevy/issues/69
         if !app.is_plugin_added::<FrameTimeDiagnosticsPlugin>() {
-            app.add_plugins(FrameTimeDiagnosticsPlugin);
+            app.add_plugins(FrameTimeDiagnosticsPlugin::default());
         }
         app.insert_resource(self.config.clone())
             .add_systems(Startup, setup)

crates/bevy_diagnostic/src/frame_time_diagnostics_plugin.rs

@@ -1,4 +1,7 @@
-use crate::{Diagnostic, DiagnosticPath, Diagnostics, FrameCount, RegisterDiagnostic};
+use crate::{
+    Diagnostic, DiagnosticPath, Diagnostics, FrameCount, RegisterDiagnostic,
+    DEFAULT_MAX_HISTORY_LENGTH,
+};
 use bevy_app::prelude::*;
 use bevy_ecs::prelude::*;
 use bevy_time::{Real, Time};
@@ -8,15 +11,49 @@ use bevy_time::{Real, Time};
 /// # See also
 ///
 /// [`LogDiagnosticsPlugin`](crate::LogDiagnosticsPlugin) to output diagnostics to the console.
-#[derive(Default)]
-pub struct FrameTimeDiagnosticsPlugin;
+pub struct FrameTimeDiagnosticsPlugin {
+    /// The total number of values to keep for averaging.
+    pub max_history_length: usize,
+    /// The smoothing factor for the exponential moving average. Usually `(history_length + 1.0) / 2.0)`.
+    pub smoothing_factor: f64,
+}
+impl Default for FrameTimeDiagnosticsPlugin {
+    fn default() -> Self {
+        Self::new(DEFAULT_MAX_HISTORY_LENGTH)
+    }
+}
+impl FrameTimeDiagnosticsPlugin {
+    /// Creates a new `FrameTimeDiagnosticsPlugin` with the specified `max_history_length` and a
+    /// reasonable `smoothing_factor`.
+    pub fn new(max_history_length: usize) -> Self {
+        Self {
+            max_history_length,
+            smoothing_factor: (max_history_length as f64 + 1.0) / 2.0,
+        }
+    }
+}
 
 impl Plugin for FrameTimeDiagnosticsPlugin {
     fn build(&self, app: &mut App) {
-        app.register_diagnostic(Diagnostic::new(Self::FRAME_TIME).with_suffix("ms"))
-            .register_diagnostic(Diagnostic::new(Self::FPS))
-            .register_diagnostic(Diagnostic::new(Self::FRAME_COUNT).with_smoothing_factor(0.0))
-            .add_systems(Update, Self::diagnostic_system);
+        app.register_diagnostic(
+            Diagnostic::new(Self::FRAME_TIME)
+                .with_suffix("ms")
+                .with_max_history_length(self.max_history_length)
+                .with_smoothing_factor(self.smoothing_factor),
+        )
+        .register_diagnostic(
+            Diagnostic::new(Self::FPS)
+                .with_max_history_length(self.max_history_length)
+                .with_smoothing_factor(self.smoothing_factor),
+        )
+        // An average frame count would be nonsensical, so we set the max history length
+        // to zero and disable smoothing.
+        .register_diagnostic(
+            Diagnostic::new(Self::FRAME_COUNT)
+                .with_smoothing_factor(0.0)
+                .with_max_history_length(0),
+        )
+        .add_systems(Update, Self::diagnostic_system);
     }
 }
 

examples/diagnostics/enabling_disabling_diagnostic.rs

@@ -12,7 +12,7 @@ fn main() {
     App::new()
         .add_plugins((
             DefaultPlugins,
-            FrameTimeDiagnosticsPlugin,
+            FrameTimeDiagnosticsPlugin::default(),
             LogDiagnosticsPlugin::default(),
         ))
         .add_systems(

examples/diagnostics/log_diagnostics.rs

@@ -10,7 +10,7 @@ fn main() {
         .add_plugins((
             DefaultPlugins,
             // Adds frame time diagnostics
-            FrameTimeDiagnosticsPlugin,
+            FrameTimeDiagnosticsPlugin::default(),
             // Adds a system that prints diagnostics to the console
             LogDiagnosticsPlugin::default(),
             // Any plugin can register diagnostics. Uncomment this to add an entity count diagnostics:

examples/stress_tests/bevymark.rs

@@ -142,7 +142,7 @@ fn main() {
                 }),
                 ..default()
             }),
-            FrameTimeDiagnosticsPlugin,
+            FrameTimeDiagnosticsPlugin::default(),
             LogDiagnosticsPlugin::default(),
         ))
         .insert_resource(WinitSettings {

examples/stress_tests/many_animated_sprites.rs

@@ -21,7 +21,7 @@ fn main() {
         // Since this is also used as a benchmark, we want it to display performance data.
         .add_plugins((
             LogDiagnosticsPlugin::default(),
-            FrameTimeDiagnosticsPlugin,
+            FrameTimeDiagnosticsPlugin::default(),
             DefaultPlugins.set(WindowPlugin {
                 primary_window: Some(Window {
                     present_mode: PresentMode::AutoNoVsync,

examples/stress_tests/many_buttons.rs

@@ -73,7 +73,7 @@ fn main() {
             }),
             ..default()
         }),
-        FrameTimeDiagnosticsPlugin,
+        FrameTimeDiagnosticsPlugin::default(),
         LogDiagnosticsPlugin::default(),
     ))
     .insert_resource(WinitSettings {

examples/stress_tests/many_components.rs

@@ -146,7 +146,7 @@ fn stress_test(num_entities: u32, num_components: u32, num_systems: u32) {
     app.add_plugins(MinimalPlugins)
         .add_plugins(DiagnosticsPlugin)
         .add_plugins(LogPlugin::default())
-        .add_plugins(FrameTimeDiagnosticsPlugin)
+        .add_plugins(FrameTimeDiagnosticsPlugin::default())
         .add_plugins(LogDiagnosticsPlugin::filtered(vec![DiagnosticPath::new(
             "fps",
         )]));

examples/stress_tests/many_cubes.rs

@@ -111,7 +111,7 @@ fn main() {
                 }),
                 ..default()
             }),
-            FrameTimeDiagnosticsPlugin,
+            FrameTimeDiagnosticsPlugin::default(),
             LogDiagnosticsPlugin::default(),
         ))
         .insert_resource(WinitSettings {

examples/stress_tests/many_foxes.rs

@@ -51,7 +51,7 @@ fn main() {
                 }),
                 ..default()
             }),
-            FrameTimeDiagnosticsPlugin,
+            FrameTimeDiagnosticsPlugin::default(),
             LogDiagnosticsPlugin::default(),
         ))
         .insert_resource(WinitSettings {