fix(tracing): wrap error with synthetic mechanism only if attaching stacktrace

[lcian]

Description

With this change, we wrap an Error captured with e.g. tracing::error with the synthetic mechanism for tracing only if we effectively have a stack trace to be reported in the error, i.e. feature backtrace is enabled and attach_stacktrace is enabled.

Without this change, using tracing::error!(error = <err>, <message>) without the backtrace feature enabled would result in Sentry issues with <unknown> as title, as shown in #723 (comment).

Closes #723

Example

fn a() {
    let err = "NaN".parse::<u32>().unwrap_err();
    tracing::error!(error = std_dyn_err(&err), "error with error");
}

fn b() {
    let err = File::open("non_existent_file.txt").unwrap_err();
    tracing::error!(error = std_dyn_err(&err), "error with error");
}

fn c() {
    let err = File::open("non_existent_file.txt").unwrap_err();
    sentry::capture_error(&err);
}

Before

After

b and c will now be grouped under a single issue. This makes sense as they are carrying an equivalent Error. In absence of stack frames, we cannot distinguish between the two.
b will still include the additional message, the mechanism: tracing indicator and the Rust Tracing Context.

While c will not include this extra info as usual.

[Swatinem]

🎉 nice! this has bothered me for a long time for the internal symbolicator errors we have. I remember this broke at some point because of messing with these fields, and we never figured out how to fix that.