tracing-subscriber 0.3.10

This release adds several new features, including a Filter implementation and
new builder API for EnvFilter, support for using a Vec<L> where L: Layer as
a Layer, and a number of smaller API improvements to make working with dynamic
and reloadable layers easier.

Added

• registry: Implement Filter for EnvFilter, allowing it to be used with
  per-layer filtering (#1983)
• registry: Filter::on_new_span, Filter::on_enter,
  Filter::on_exit, Filter::on_close and Filter::on_record callbacks to
  allow Filters to track span states internally (#1973, #2017, #2031)
• registry: Filtered::filter and Filtered::filter_mut accessors
  (#1959)
• registry: Filtered::inner and Filtered::inner_mut accessors to borrow
  the wrapped Layer (#2034)
• layer: Implement Layer for Vec<L: Layer>, to allow composing together
  a dynamically sized list of Layers (#2027)
• layer: Layer::boxed method to make type-erasing Layers easier
  (#2026)
• fmt: fmt::Layer::writer and fmt::Layer::writer_mut accessors (#2034)
• fmt: fmt::Layer::set_ansi method to allow changing the ANSI formatting
  configuration at runtime (#2034)
• env-filter: EnvFilter::builder to configure a new EnvFilter prior to
  parsing it (#2035)
• Several documentation fixes and improvements (#1972, #1971, #2023,
  #2023)

Fixed

• fmt: fmt::Layer's auto traits no longer depend on the Subscriber type
  parameter's auto traits (#2025)
• env-filter: Fixed missing help text when the ansi feature is disabled
  (#2029)

Thanks to new contributors @TimoFreiberg and @wagenet, as well as @CAD97 for
contributing to this release!