subscriber: prepare to release 0.2.4

[hawkw]

0.2.4 (April 6, 2020)

This release includes several API ergonomics improvements, including shorthand
constructors for many types, and an extension trait for initializing subscribers
using method-chaining style. Additionally, several bugs in less commonly used
fmt APIs were fixed.

Added

• fmt: Shorthand free functions for constructing most types in fmt
  (including tracing_subscriber::fmt() to return a SubscriberBuilder,
  tracing_subscriber::fmt::layer() to return a format Layer, etc) (subscriber: add more ergonomic subscriber configuration APIs #660)
• registry: Shorthand free function tracing_subscriber::registry() to
  construct a new registry (subscriber: add more ergonomic subscriber configuration APIs #660)
• Added SubscriberInitExt extension trait for more ergonomic subscriber
  initialization (subscriber: add more ergonomic subscriber configuration APIs #660)

Changed

• fmt: Moved LayerBuilder methods to Layer (subscriber: move fmt::LayerBuilder methods to fmt::Layer #655)

Deprecated

• fmt: LayerBuilder, as Layer now implements all builder methods (subscriber: move fmt::LayerBuilder methods to fmt::Layer #655)

Fixed

• fmt: Fixed Compact formatter not omitting levels with
  with_level(false) (subscriber: fix Compact formatter not honoring with_level #657)
• fmt: Fixed fmt::Layer duplicating the fields for a new span if another
  layer has already formatted its fields (Fix but in new_span for fmt layer #634)
• fmt: Added missing space when using record to add new fields to a span
  that already has fields (subscriber: add space when recording new values on an existing span #659)
• Updated outdated documentation (subscriber: fix outdated documentation #647)