Function flexi_logger::opt_format

pub fn opt_format(w: &mut dyn Write, record: &Record<'_>) -> Result<(), Error>

A logline-formatter that produces log lines like
[2016-01-13 15:25:01.640870 +01:00] INFO [src/foo/bar:26] Task successfully read from conf.json
i.e. with timestamp and file location.