### Added

- **field**: `field::Empty` type for declaring empty fields whose values
  will be recorded later (#548)
- **field**: `field::Value` implementations for `Wrapping` and `NonZero*`
  numbers (#538)
- **attributes**: Support for adding arbitrary literal fields to spans
  generated by `#[instrument]` (#569)
- **attributes**: `#[instrument]` now emits a helpful compiler error when
  attempting to skip a function parameter (#600)

### Changed

- **attributes**: The `#[instrument]` attribute was placed under an
  on-by-default  feature flag "attributes" (#603)

### Fixed

- Broken and unresolvable links in RustDoc (#595)

Thanks to @oli-cosmian and @Kobzol for contributing to this release!

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