[tracing-subscriber][tracing] improve duplciate tracing situation a bit

[xMAC94x]

Motivation

#1372 motived me, but sadly this is NOT a fix. just a warning.
I raised a question in discord and tried to improve the situation, however I figured out I couln't fix in easily, so at least i wanted to warn about it.

Solution

While coding I removed some implicit Deref usages, while nothing is wrong with it. I think that we prob stick to https://doc.rust-lang.org/std/ops/trait.Deref.html#when-to-implement-deref-or-derefmut and should remove this in a 0.4 at some point.

The warning to tracing is kept short, linking to tracing-subscriber because realistically most people are gonna use that and are suprised when tracing-subscriber doesn't follow what tracing promises.

[mladedav]

Suggested change

	/// where it duplicates span fiels because of its implementation. See details at: <a href="https://github.com/tokio-rs/tracing/issues/1372">Github</a>
	/// where it duplicates span fields because of its implementation. See details at: <a href="https://github.com/tokio-rs/tracing/issues/1372">Github</a>

[xMAC94x]

fixed

[mladedav]

This is definitely not a good thing for this note, just beforee it is another note which ends with : because right after it was supposed to be an example illustrating what the note was about. So you might want to move this further down.

Also this bug happens specifically only when someone uses multiple fmt layers, right? I think that's pretty important to mention, the note sounds like this happens every time but most users will never encounter this.

[xMAC94x]

//[dependencies]
//tracing-subscriber = "0.3"
//tracing = "0.1"


fn main() {
    tracing_subscriber::fmt()
        .with_max_level(tracing::Level::TRACE)
        .init();

    let span = tracing::info_span!("request", duplicate = tracing::field::Empty);

    span.record("duplicate", &100);
    span.record("duplicate", &200);
    span.record("duplicate", tracing::field::Empty);
    span.record("duplicate", &300);

    // produces:
    // 2025-06-19T10:45:31.070903Z  INFO request{duplicate=100 duplicate=200  duplicate=300}: bbbb: some event!
    //                                           ^ first duplicate
    //                                                         ^ second duplcate
    //                                                                      ^ an additional space by the empty field
    //                                                                        ^ expected value here

    let _guard = span.enter();

    tracing::info!("some event!");
}

[xMAC94x]

This is an other minimal example it can trigger the bug. no need for fancy layers.
It just appends... always to a simple String:
https://github.com/tokio-rs/tracing/blob/tracing-subscriber-0.3.19/tracing-subscriber/src/fmt/format/mod.rs#L246-L249

I moved the note to the bottom to not interfere with the :

Though in my code I also had multiple layers, because I think one will eventually need it when writing enterprise software.