feature: add additional fields with #[instrument(err)]

[nrdxp]

Feature Request

It would be nice if we could add additional fields to the event created when using #[instrument(err)], as it stands now, unless I'm missing something obvious, it is limited only to the error field, which makes attaching metadata using this pattern impossible.

Crates

tracing-attributes

Motivation

For example, I have an error that returns a detected path on the system where the error originated from, I'd like to log this in a seperate path field, but in order to do that, I have to write the code manually with tracing::error! instead of relying on the more declarative #[instrument(err)]

Proposal

It would be pretty nice if the fields of a struct were just automatically printed as extra fields in the log, or perhaps you could select fields of a struct inside the macro, e.g. #[instrument(err(fields = ["log", "these", "fields"]))]

Of you could do something a bit more manual and provide a reference to the error to construct fields:
#[instrument(err(fields(path = err.path)) where err is a reference to the error.

Or maybe just a combination of the two, where if the = is omitted, then the field is assumed to exist inside the err reference:
#[instrument(err(fields(path, some = err.other_field))

Alternatives

Maybe this is already possible and I just missed it? If so, some instruction would be much appreciated 🙏