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## Motivation Allow filter layers to filter on the contents of events (see #2007). ## Solution This branch adds a new `Subscriber::event_enabled` method, taking an `Event` and returning `bool`. This is called before the `Subscriber::event` method, and if it returns `false`, `Subscriber::event` is not called. For simple subscriber (e.g. not using `Layer`s), the `event_enabled` method is not particulary necessary, as the subscriber can just skip the `event` call. However, this branch also adds a new `Layer::event_enabled` method, with the signature: ```rust fn event_enabled(&self, event: &Event<'_>, ctx: Context<'_, S>) -> bool; ``` This is called before `Layer::on_event`, and if `event_enabled` returns `false`, `on_event` is not called (nor is `Subscriber::event`). This allows filter `Layer`s to filter out an `Event` based on its fields. Closes #2007