tracing-subscriber 0.3.16

This release of tracing-subscriber fixes a regression introduced in
v0.3.15 where Option::None's Layer implementation would
set the max level hint to OFF. In addition, it adds several new APIs,
including the Filter::event_enabled method for filtering events based on
fields values, and the ability to log internal errors that occur when writing a
log line.

This release also replaces the dependency on the unmaintained [ansi-term]
crate with the [nu-ansi-term] crate, resolving an informational security
advisory (RUSTSEC-2021-0139) for [ansi-term]'s maintainance status. This
increases the minimum supported Rust version (MSRV) to Rust 1.50+, although the
crate should still compile for the previous MSRV of Rust 1.49+ when the ansi
feature is not enabled.

Fixed

• layer: Option::None's Layer impl always setting the max_level_hint
  to LevelFilter::OFF (#2321)
• Compilation with -Z minimal versions (#2246)
• env-filter: Clarify that disabled level warnings are emitted by
  tracing-subscriber (#2285)

Added

• fmt: Log internal errors to stderr if writing a log line fails (#2102)
• fmt: FmtLayer::log_internal_errors and
  FmtSubscriber::log_internal_errors methods for configuring whether internal
  writer errors are printed to stderr (#2102)
• fmt: #[must_use] attributes on builders to warn if a Subscriber is
  configured but not set as the default subscriber (#2239)
• filter: Filter::event_enabled method for filtering an event based on its
  fields (#2245, #2251)
• filter: Targets::default_level accessor ([#2242])

Changed

• ansi: Replaced dependency on unmaintained ansi-term crate with
  nu-ansi-term ((#2287, fixes informational advisory RUSTSEC-2021-0139)
• tracing-core: updated to 0.1.30
• Minimum Supported Rust Version (MSRV) increased to Rust 1.50+ (when the
  ansi) feature flag is enabled (#2287)

Documented

• fmt: Correct inaccuracies in fmt::init documentation (#2224)
• filter: Fix incorrect doc link in filter::Not combinator (#2249)

Thanks to new contributors @cgbur, @DesmondWillowbrook, @RalfJung, and
@poliorcetics, as well as returning contributors @CAD97, @connec, @jswrenn,
@guswynn, and @bryangarza, for contributing to this release!