// sourced adds a source field to the logger that contains
// the file name and line where the logging happened.
func (l logger) sourced() *logrus.Entry {
	_, file, line, ok := runtime.Caller(2)
	if !ok {
		file = "<???>"
		line = 1
	} else {
		slash := strings.LastIndex(file, "/")
		file = file[slash+1:]
	}
	return l.entry.WithField("source", fmt.Sprintf("%s:%d", file, line))
}