Marks this field as the captured span trace, surfaced via
`Diagnostic::oopsie_spantrace`. The field's type must have `SpanTrace` as its
last path segment; it is auto-captured (and excluded from the selector). Marking
a field is only needed when its type isn't recognized automatically.
Forms: `spantrace`, `spantrace = true`, `spantrace = false`.
### Example
```
use oopsie::SpanTrace;
#[oopsie::oopsie]
#[oopsie(module(false))]
pub enum AppError {
#[oopsie("boom")]
Boom {
#[oopsie(spantrace)]
st: SpanTrace,
},
}
let err = Boom.build();
assert_eq!(err.to_string(), "boom");
```