tracing 0.1.30

This release adds experimental support for recording structured field
values using the valuable crate. See this blog post for
details on valuable.

Note that valuable support currently requires --cfg tracing_unstable. See
the documentation for details.

This release also adds a new enabled! macro for testing if a span or event
would be enabled.

Added

• field: Experimental support for recording field values using the
  valuable crate (#1608, #1888, #1887)
• enabled! macro for testing if a span or event is enabled (#1882)

Changed

• tracing-core: updated to 0.1.22
• tracing-attributes: updated to 0.1.19

Fixed

• log: Fixed "use of moved value" compiler error when the "log" feature is
  enabled (#1823)
• Fixed macro hygiene issues when used in a crate that defines its own concat!
  macro (#1842)
• A very large number of documentation fixes and improvements.

Thanks to @@vlad-scherbina, @Skepfyr, @Swatinem, @guswynn, @teohhanhui,
@xd009642, @tobz, @d-e-s-o@0b01, and @nickelc for contributing to this release!