Add support for updating the tracing subscriber in LogPlugin (#10822)

# Objective

This PR is heavily inspired by
#7682
It aims to solve the same problem: allowing the user to extend the
tracing subscriber with extra layers.

(in my case, I'd like to use `use
metrics_tracing_context::{MetricsLayer, TracingContextLayer};`)


## Solution

I'm proposing a different api where the user has the opportunity to take
the existing `subscriber` and apply any transformations on it.

---

## Changelog

- Added a `update_subscriber` option on the `LogPlugin` that lets the
user modify the `subscriber` (for example to extend it with more tracing
`Layers`


## Migration Guide

> This section is optional. If there are no breaking changes, you can
delete this section.

- Added a new field `update_subscriber` in the `LogPlugin`

---------

Co-authored-by: Charles Bournhonesque <cbournhonesque@snapchat.com>

Cargo.toml

@@ -1035,6 +1035,16 @@ description = "Illustrate how to use generate log output"
 category = "Application"
 wasm = true
 
+[[example]]
+name = "log_layers"
+path = "examples/app/log_layers.rs"
+
+[package.metadata.example.log_layers]
+name = "Log layers"
+description = "Illustrate how to add custom log layers"
+category = "Application"
+wasm = false
+
 [[example]]
 name = "plugin"
 path = "examples/app/plugin.rs"

crates/bevy_log/src/lib.rs

@@ -38,8 +38,10 @@ pub use bevy_utils::tracing::{
     debug, debug_span, error, error_span, info, info_span, trace, trace_span, warn, warn_span,
     Level,
 };
+pub use tracing_subscriber;
 
 use bevy_app::{App, Plugin};
+use bevy_utils::tracing::Subscriber;
 use tracing_log::LogTracer;
 #[cfg(feature = "tracing-chrome")]
 use tracing_subscriber::fmt::{format::DefaultFields, FormattedFields};
@@ -64,6 +66,7 @@ use tracing_subscriber::{prelude::*, registry::Registry, EnvFilter};
 ///         .add_plugins(DefaultPlugins.set(LogPlugin {
 ///             level: Level::DEBUG,
 ///             filter: "wgpu=error,bevy_render=info,bevy_ecs=trace".to_string(),
+///             update_subscriber: None,
 ///         }))
 ///         .run();
 /// }
@@ -100,13 +103,21 @@ pub struct LogPlugin {
     /// Filters out logs that are "less than" the given level.
     /// This can be further filtered using the `filter` setting.
     pub level: Level,
+
+    /// Optionally apply extra transformations to the tracing subscriber.
+    /// For example add [`Layers`](tracing_subscriber::layer::Layer)
+    pub update_subscriber: Option<fn(BoxedSubscriber) -> BoxedSubscriber>,
 }
 
+/// Alias for a boxed [`Subscriber`].
+pub type BoxedSubscriber = Box<dyn Subscriber + Send + Sync + 'static>;
+
 impl Default for LogPlugin {
     fn default() -> Self {
         Self {
             filter: "wgpu=error,naga=warn".to_string(),
             level: Level::INFO,
+            update_subscriber: None,
         }
     }
 }
@@ -179,7 +190,11 @@ impl Plugin for LogPlugin {
             #[cfg(feature = "tracing-tracy")]
             let subscriber = subscriber.with(tracy_layer);
 
-            finished_subscriber = subscriber;
+            if let Some(update_subscriber) = self.update_subscriber {
+                finished_subscriber = update_subscriber(Box::new(subscriber));
+            } else {
+                finished_subscriber = Box::new(subscriber);
+            }
         }
 
         #[cfg(target_arch = "wasm32")]

examples/README.md

@@ -171,6 +171,7 @@ Example | Description
 [Empty](../examples/app/empty.rs) | An empty application (does nothing)
 [Empty with Defaults](../examples/app/empty_defaults.rs) | An empty application with default plugins
 [Headless](../examples/app/headless.rs) | An application that runs without default plugins
+[Log layers](../examples/app/log_layers.rs) | Illustrate how to add custom log layers
 [Logs](../examples/app/logs.rs) | Illustrate how to use generate log output
 [No Renderer](../examples/app/no_renderer.rs) | An application that runs with default plugins and displays an empty window, but without an actual renderer
 [Plugin](../examples/app/plugin.rs) | Demonstrates the creation and registration of a custom plugin

examples/app/log_layers.rs

@@ -0,0 +1,47 @@
+//! This example illustrates how to add custom log layers in bevy.
+
+use bevy::{
+    log::tracing_subscriber::{layer::SubscriberExt, Layer},
+    log::BoxedSubscriber,
+    prelude::*,
+    utils::tracing::Subscriber,
+};
+
+struct CustomLayer;
+
+impl<S: Subscriber> Layer<S> for CustomLayer {
+    fn on_event(
+        &self,
+        event: &bevy::utils::tracing::Event<'_>,
+        _ctx: bevy::log::tracing_subscriber::layer::Context<'_, S>,
+    ) {
+        println!("Got event!");
+        println!("  level={:?}", event.metadata().level());
+        println!("  target={:?}", event.metadata().target());
+        println!("  name={:?}", event.metadata().name());
+    }
+}
+
+fn update_subscriber(subscriber: BoxedSubscriber) -> BoxedSubscriber {
+    Box::new(subscriber.with(CustomLayer))
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins.set(bevy::log::LogPlugin {
+            update_subscriber: Some(update_subscriber),
+            ..default()
+        }))
+        .add_systems(Update, log_system)
+        .run();
+}
+
+fn log_system() {
+    // here is how you write new logs at each "log level" (in "most import" to
+    // "least important" order)
+    error!("something failed");
+    warn!("something bad happened that isn't a failure, but thats worth calling out");
+    info!("helpful information that is worth printing by default");
+    debug!("helpful for debugging");
+    trace!("very noisy");
+}

examples/ecs/system_piping.rs

@@ -14,6 +14,7 @@ fn main() {
         .add_plugins(LogPlugin {
             level: Level::TRACE,
             filter: "".to_string(),
+            ..default()
         })
         .add_systems(
             Update,