Add example demonstrating how to enable / disable diagnostics (#14741)

# Objective

fixes #14569

## Solution

added an example to the diagnostic examples and linked the code to the
docs of the diagnostic library itself.

## Testing

I tested locally on my laptop in a web browser. Looked fine. You are
able to collapse the whole "intro" part of the doc to get to the links
sooner (for those who may think that including the example code here is
annoying to scroll through)

I would like people to run ```cargo doc``` and go the bevy_diagnostic
page to see if they have any issues or suggestions.

---

## Showcase

<img width="1067" alt="Screenshot 2024-08-14 at 12 52 16"
src="https://github.com/user-attachments/assets/70b6c18a-0bb9-4656-ba53-c416f62c6116">

---------

Co-authored-by: dpeke <dpekelis@funstage.com>

Cargo.toml

@@ -1604,6 +1604,17 @@ description = "Shows how to create a custom diagnostic"
 category = "Diagnostics"
 wasm = true
 
+[[example]]
+name = "enabling_disabling_diagnostic"
+path = "examples/diagnostics/enabling_disabling_diagnostic.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.enabling_disabling_diagnostic]
+name = "Enabling/disabling diagnostic"
+description = "Shows how to disable/re-enable a Diagnostic during runtime"
+category = "Diagnostics"
+wasm = true
+
 # ECS (Entity Component System)
 [[example]]
 name = "ecs_guide"

examples/README.md

@@ -265,6 +265,7 @@ Example | Description
 Example | Description
 --- | ---
 [Custom Diagnostic](../examples/diagnostics/custom_diagnostic.rs) | Shows how to create a custom diagnostic
+[Enabling/disabling diagnostic](../examples/diagnostics/enabling_disabling_diagnostic.rs) | Shows how to disable/re-enable a Diagnostic during runtime
 [Log Diagnostics](../examples/diagnostics/log_diagnostics.rs) | Add a plugin that logs diagnostics, like frames per second (FPS), to the console
 
 ## ECS (Entity Component System)

examples/diagnostics/enabling_disabling_diagnostic.rs

@@ -0,0 +1,30 @@
+//! Shows how to disable/re-enable a Diagnostic during runtime
+
+use std::time::Duration;
+
+use bevy::{
+    diagnostic::{DiagnosticsStore, FrameTimeDiagnosticsPlugin, LogDiagnosticsPlugin},
+    prelude::*,
+    time::common_conditions::on_timer,
+};
+
+fn main() {
+    App::new()
+        .add_plugins((
+            DefaultPlugins,
+            FrameTimeDiagnosticsPlugin,
+            LogDiagnosticsPlugin::default(),
+        ))
+        .add_systems(
+            Update,
+            toggle.run_if(on_timer(Duration::from_secs_f32(10.0))),
+        )
+        .run();
+}
+
+fn toggle(mut store: ResMut<DiagnosticsStore>) {
+    for diag in store.iter_mut() {
+        info!("toggling diagnostic {}", diag.path());
+        diag.is_enabled = !diag.is_enabled;
+    }
+}