* subscriber: prepare to release 0.2.0-alpha.3

### Added

- **fmt**: Public `FormattedFields::new` constructor (#478)
- **fmt**: Added examples to `fmt::Layer` documentation (#510)
- Documentation now shows what feature flags are required by each API item (#525)

### Fixed

- **fmt**: Missing space between timestamp and level (#480)
- **fmt**: Incorrect formatting with `with_target(false)` (#481)
- **fmt**: `fmt::SubscriberBuilder::init` not setting up `log` compatibility
  (#489)
- **registry**: Spans exited out of order not being closed properly on exit
  (#509)
- **registry**: Memory leak when spans are closed by a child span closing (#514)
- **registry**: Spans closed by a child span closing not also closing _their_
  parents (#514)
- Compilation errors with `no-default-features` (#499, #500)

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