tracing-attributes 0.1.21

This release adds support for setting explicit parent and follows-from spans
in the #[instrument] attribute.

Added

• #[instrument(follows_from = ...)] argument for setting one or more
  follows-from span (#2093)
• #[instrument(parent = ...)] argument for overriding the generated span's
  parent (#2091)

Fixed

• Extra braces around async blocks in expanded code (causes a Clippy warning)
  (#2090)
• Broken documentation links (#2068, #2077)

Thanks to @jarrodldavis, @ben0x539, and new contributor @jswrenn for
contributing to this release!