tracing: prepare to release v0.1.38

[hawkw]

0.1.38 (April 25th, 2023)

This tracing release changes the Drop implementation for
Instrumented Futures so that the attached Span is entered when
dropping the Future. This means that events emitted by the Future's
Drop implementation will now be recorded within its Span. It also
adds #[inline] hints to methods called in the event! macro's
expansion, for an improvement in both binary size and performance.

Additionally, this release updates the tracing-attributes dependency
to v0.1.24, which updates the syn dependency to
v2.x.x. tracing-attributes v0.1.24 also includes improvements to the
#[instrument] macro; see the tracing-attributes 0.1.24 release
notes for details.

Added

• Instrumented futures will now enter the attached Span in their
  Drop implementation, allowing events emitted when dropping the
  future to occur within the span (tracing, tracing-futures: instrument Future inside Drop #2562)
• #[inline] attributes for methods called by the event! macros,
  making generated code smaller (inline several methods to make tracing::event! smaller #2555)
• attributes: level argument to #[instrument(err)] and
  #[instrument(ret)] to override the level of the generated return
  value event (attributes: add support for custom levels for ret and err (#2330) #2335)
• attributes: Improved compiler error message when #[instrument]
  is added to a const fn (Add compile error on const fn instrumentation #2418)

Changed

• tracing-attributes: updated to 0.1.24
• Removed unneeded cfg-if dependency (Remove dep cfg-if from tracing #2553)
• attributes: Updated syn dependency to 2.0 (attributes: update to syn 2.0 #2516)

Fixed

• attributes: Fix clippy::unreachable warnings in
  #[instrument]-generated code (tracing-attributes: allow warning clippy unreachable #2356)
• attributes: Removed unused "visit" feature flag from syn
  dependency (tracing-attributes: remove unused syn's feature visit #2530)

Documented

• attributes: Documented default level for #[instrument(err)]
  (attributes: document default level for err #2433)
• attributes: Improved documentation for levels in #[instrument]
  (Better documentation & tests for using Level constants for #[instrument(level = X)] #2350)

Thanks to @nitnelave, @jsgf, @Abhicodes-crypto, @LukeMathWalker,
@andrewpollack, @quad, @klensy, @davidpdrsn, @dbidwell94, @ldm0,
@NobodyXu, @ilsv, and @daxpedda for contributing to this release!