This release adds experimental support for recording structured field
values using the valuable crate to the format::Json formatter. In
particular, user-defined types which are recorded using their
valuable::Valuable implementations will be serialized as JSON objects,
rather than using their fmt::Debug representation. See this blog post
for details on valuable.

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

Additionally, this release includes a number of other smaller API improvements.

Added

• json: Experimental support for recording valuable values as structured
  JSON (#1862, #1901)
• filter: Targets::would_enable method for testing if a Targets filter
  would enable a given target (#1903)
• fmt: map_event_format, map_fmt_fields, and map_writer methods to
  fmt::Layer and fmt::SubscriberBuilder (#1871)

Changed

• tracing-core: updated to 0.1.22

Fixed

• Set smallvec minimal version to 1.2.0, to fix compilation errors with -Z minimal-versions (#1890)
• Minor documentation fixes (#1902, #1893)

Thanks to @guswynn, @glts, and @lilyball for contributing to this release!