tracing-subscriber 0.3.12

This release of tracing-subscriber adds a new Layer::event_enabled method,
which allows Layers to filter events after their field values are recorded;
a Filter implementation for reload::Layer, to make using reload with
per-layer filtering more ergonomic, and additional inherent method downcasting
APIs for the Layered type. In addition, it includes dependency updates, and
minor fixes for documentation and feature flagging.

Added

• layer: Layer::event_enabled method, which can be implemented to filter
  events based on their field values (#2008)
• reload: Filter implementation for reload::Layer (#2159)
• layer: Layered::downcast_ref and Layered::is inherent methods
  (#2160)

Changed

• parking_lot: Updated dependency on parking_lot to 0.13.0 (#2143)
• Replaced lazy_static dependency with once_cell ([#2147])

Fixed

• Don't enable tracing-core features by default (#2107)
• Several documentation link and typo fixes (#2064, #2068, #[2077], #2161,
  #1088)

Thanks to @ben0x539, @jamesmunns, @georgemp, @james7132, @jswrenn, @CAD97, and
@guswynn for contributing to this release!