### Added:

- **env_filter**: Documentation for filtering directives (#554)
- **registry**, **env_filter**: Updated `smallvec` dependency to 0.1
  (#543)

### Fixed:

- **registry**: Fixed a memory leak in the slab used to store per-span
  data (3c35048)
- **Layer**: Fixed `Layered` subscribers failing to downcast to their
  own type (#549)
- **fmt**: Fixed a panic when multiple layers insert `FormattedFields`
  extensions from the same formatter type (1c3bb70)
- **fmt**: Fixed `fmt::Layer::on_record` inserting a new
  `FormattedFields` when  formatted fields for a span already exist
  (1c3bb70)

Signed-off-by: Eliza Weisman <eliza@buoyant.io>