tracing-core: correctly use features for tracing macro

[mladedav]

Motivation

An alternative to #2655.

Currently, tracing-core defines a tracing feature which is never used, because it was intended only for expansion inside a macro, but the #[cfg(feature = "tracing")] attribute is expanded and is evaluated based on whether the consuming crate (in this case either axum or axum-extra) has the feature enabled.

Solution

Based on the feature, we can either expand the macro to a tracing::event! call or we can just expand to nothing. This way, the feature works as intended.

One difference between this and removing the feature is that with this change, if either tracing or tracing-extra enables the tracing feature, it will transitively enable the same feature in tracing-core so extractors from both crates will have their rejections logged.

If we instead remove the feature from tracing-core, each crate's tracing feature will be independent which may result in some extractors being logged and others not. So although it gives more control to the user, I personally think the behavior may be surprising.

Also, removing the feature is a breaking change while this is not.

[jplatte]

Hmm, I get where you're coming from re. confusion but actually I find it also pretty weird that right now, we use this tracing target for both axum and axum-extra extractors... I'm wondering if it wouldn't be better to just copy-paste the macro into each crate.