tracing-subscriber 0.3.4

This release contains bugfixes for the fmt module, as well as documentation
improvements.

Fixed

• fmt: Fixed fmt not emitting log lines when timestamp formatting fails
  (#1689)
• fmt: Fixed double space before thread IDs with Pretty formatter
  (#1778)
• Several documentation improvements (#1608, #1699, #1701)

Thanks to new contributors @Swatinem and @rukai for contributing to this
release!