Add span and event macros that are only enabled in debug mode

[hawkw]

See also tokio-rs/tokio#827 (review).

It would be nice to have debug_span! and debug_event! macros in the tokio-trace-macros crate which are only enabled in debug mode builds.

We can detect debug mode with

#[cfg(debug_assertions)]

In debug mode, these macros would behave identically to the tokio_trace crate's span! and debug! macros. In release mode (i.e. #[cfg(not(debug_assertions))]) the debug_span! macro would always evaluate to a disabled span struct (Span::new_disabled()) while debug_event! would do nothing.

[hawkw]

tokio-rs/tokio#959 is a proposed change to the tokio-trace crate that would obsolete this.

[hawkw]

Closed by tokio-rs/tokio#987.