Function fmt_diag 

pub fn fmt_diag<'a, T>(f: &mut dyn Write, container: &'a T) -> Result<(), Error>
where
    T: ErrorDiagnostic + AsError,
    ResultType: From<&'a <T as AsError>::Target>,

Formats a full diagnostic view of an error for logging and tracing.

Includes the result type, signature, tag, service, and the display representation of the error and its source chain. For ServiceError results, a resolved backtrace is appended when available.