tracing-subscriber 0.3.18

This release of tracing-subscriber adds support for the NO_COLOR environment
variable (an informal standard to disable emitting ANSI color escape codes) in
fmt::Layer, reintroduces support for the chrono crate, and increases the
minimum supported Rust version (MSRV) to Rust 1.63.0.

It also introduces several minor API improvements.

Added

• chrono: Add chrono implementations of FormatTime (#2690)
• subscriber: Add support for the NO_COLOR environment variable in
  fmt::Layer (#2647)
• fmt: make format::Writer::new() public (#2680)
• filter: Implement layer::Filter for Option<Filter> (#2407)

Changed

• log: bump version of tracing-log to 0.2 (#2772)
• Increased minimum supported Rust version (MSRV) to 1.63.0+.

Thanks to @shayne-fletcher, @dmlary, @kaifastromai, and @jsgf for contributing!