Add deprecation warning when using the `tracing::instrument` macro on a trait method without a body

[CaptainTux]

Based on a build error I found and a discussion on the Rust community discord server, I believe it would be good to add a deprecation warning, or a proper error, when using the tracing::instrument macro on a trait method without a body, which used to work.
In my case the error message was not properly identifying the problem, and I only learned about the solution in the discord server where a person probably already knew about this change problem.

[konnorandrews]

For clarification, before tracing-attributes v0.1.29 the instrument macro allowed being used on trait functions without bodies. The macro would generate an empty body for the trait's function (ignoring the code for instrumenting).

trait Demo {
    #[tracing::instrument]
    fn demo(); // generated code has a default body
}

This was likely an unintended result as it causes a semver hazard because it changes the definition of the trait.