Improve tracing layer customization

[cBournhonesque]

Objective

• Fixes LogPlugin's update_subscriber is broken when adding extra FmtLayers #12597

The current tracing customization option (the update_subscriber field) was basically unusable because it provides a dyn Subscriber and most layers require a Subscriber that also implements for<'a> LookupSpan<'a, Data=Data<'a>>, so it was impossible to add a layer on top of the dyn Subscriber.

This PR provides an alternative way of adding additional tracing layers to the LogPlugin by instead creating an Option<Layer>.

This is enough for most situations because Option<Layer> and Vec<Layer> both implement Layer.

Solution

• Replace the update_subscriber field of LogPlugin with a custom_layer field which is function pointer returning an Option<BoxedLayer>
• Update the examples to showcase that this works:
  • with multiple additional layers
  • with Layers that were previously problematic, such as bevy::log::tracing_subscriber::fmt::layer().with_file(true) (mentioned in the issue)

Note that in the example this results in duplicate logs, since we have our own layer on top of the default fmt_layer added in the LogPlugin; maybe in the future we might want to provide a single one? Or to let the user customize the default fmt_layer ? I still think this change is an improvement upon the previous solution, which was basically broken.

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Changelog

This section is optional. If this was a trivial fix, or has no externally-visible impact, you can delete this section.

• The LogPlugin's update_subscriber field has been replaced with custom_layer to allow the user to flexibly add a tracing::Layer to the layer stack

Migration Guide

• The LogPlugin's update_subscriber field has been replaced with custom_layer

[BD103]

In general looks good! I'm a bit concerned about encouraging people to create Box<Vec<...>>, but since the function itself only gets called once it's probably fine.