tracing 0.1.27

This release adds a new Span::or_current method to aid in
efficiently propagating span contexts to spawned threads or tasks.
Additionally, it updates the tracing-core version to 0.1.20 and
the tracing-attributes version to 0.1.16, ensuring that a number
of new features in those crates are present.

Fixed

• instrument: Added missing WithSubscriber implementations for
  futures and other types (#1424)

Added

• Span::or_current method, to help with efficient span context
  propagation (#1538)
• attributes: add skip_all option to #[instrument] (#1548)
• attributes: record primitive types as primitive values rather than
  as fmt::Debug (#1378)
• core: NoSubscriber, a no-op Subscriber implementation
  (#1549)
• core: Added Visit::record_f64 and support for recording
  floating-point values (#1507, #1522)
• A large number of documentation improvements and fixes (#1369,
  #1398, #1435, #1442, #1524, #1556)

Thanks to new contributors @dzvon and @mbergkvist, as well as @teozkr,
@maxburke, @LukeMathWalker, and @jsgf, for contributing to this release!