tracing: prepare to release 0.1.18

[hawkw]

Fixed

• Fixed a bug where LevelFilter::OFF (and thus also the
  static_max_level_off feature flag) would enable all traces,
  rather than none (core: add faster fast path for level filtering #853)
• log: Fixed tracing macros and Spans not checking
  log::max_level before emitting log records (tracing: make the "log" feature honor log's max level #870)

Changed

• macros: Macros now check the global max level
  (LevelFilter::current) before the per-callsite cache when
  determining if a span or event is enabled. This significantly improves
  performance in some use cases (core: add faster fast path for level filtering #853)
• macros: Simplified the code generated by macro expansion
  significantly, which may improve compile times and/or rustc
  optimizatation of surrounding code (tracing: move macro callsite impls out of macro expansion #869, tracing: move macro callsite impls out of macro expansion #869)
• macros: Macros now check the static max level before checking any
  runtime filtering, improving performance when a span or event is
  disabled by a static_max_level_XXX feature flag (tracing: remove dispatcher::has_been_set check in macros #868)
• LevelFilter is now a re-export of the tracing_core::LevelFilter
  type, it can now be used interchangably with the versions in
  tracing-core and tracing-subscriber (core: add faster fast path for level filtering #853)
• Significant performance improvements when comparing LevelFilters and
  Levels (core: add faster fast path for level filtering #853)
• Updated the minimum tracing-core dependency to 0.1.12 (core: add faster fast path for level filtering #853)

Added

• macros: Quoted string literals may now be used as field names, to
  allow fields whose names are not valid Rust identifiers (Support literals for field names. #790)
• docs: Several documentation improvements (docs: Add tide-tracing to "Related Crates" #850, docs: Remove reference to old tokio feature flag experimental-tracing #857, readme: Add tracing-wasm to "Related Crates" #841)
• LevelFilter::current() function, which returns the highest level
  that any subscriber will enable (core: add faster fast path for level filtering #853)
• Subscriber::max_level_hint optional trait method, for setting the
  value returned by LevelFilter::current() (core: add faster fast path for level filtering #853)

Thanks to new contributors @cuviper, @ethanboxx, @ben0x539, @dignati,
@colelawrence, and @rbtcollins for helping out with this release!

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