feat(log): support customizing default log formatting

[JeanMertz]

The LogPlugin now allows overriding the default
tracing_subscriber::fmt::Layer through a new fmt_layer option. This enables customization of the default log output format without having to replace the entire logging system.

For example, to disable timestamps in the log output:

fn fmt_layer(_app: &mut App) -> Option<bevy::log::BoxedFmtLayer> {
    Some(Box::new(
        bevy::log::tracing_subscriber::fmt::Layer::default()
            .without_time()
            .with_writer(std::io::stderr),
    ))
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins.set(bevy::log::LogPlugin {
            fmt_layer,
            ..default()
        }))
        .run();
}

This is different from the existing custom_layer option, because that option adds additional layers to the subscriber, but can't modify the default formatter layer (at least, not to my knowledge).

I almost always disable timestamps in my Bevy logs, and usually also tweak other default log formatting (such as with_span_events), which made it so that I always had to disable the default logger. This allows me to use everything the Bevy logger supports (including tracy support), while still formatting the default logs the way I like them.

[JeanMertz]

I couldn't get this to work in a generic way, but if anyone can, I'd love to swap this out for something more maintainable (these type aliases would break if we changed ordering of existing layers or add additional layers).

[cBournhonesque]

ha...
This is due to

#[cfg(feature = "trace")]
let subscriber = subscriber.with(tracing_error::ErrorLayer::default());

?

Yes in general it's hard to get things working correctly with tracing

[JeanMertz]

Correct, I couldn't find a way to have a generic layer type definition that works for any stack of layers, and it has to work with certain layers included or removed, depending on the trace feature.

I've had my eye on https://github.com/fast/fastrace for a while actually (combined with https://github.com/fast/logcall and https://github.com/fast/logforth).

[cBournhonesque]

I think this works for now but in general the custom_layer and fmt_layer feel a bit hacky; it would be nice to figure out some cleaner way to override/add extra layers

[JeanMertz]

I think this works for now but in general the custom_layer and fmt_layer feel a bit hacky; it would be nice to figure out some cleaner way to override/add extra layers

I agree, but I would prefer that to be a follow-up PR.