log.go
func (l *slog) Warnf(format string, args ...interface{}) {
	lvl := l.Level()
	if lvl <= LevelWarn {
		l.b.printf("WRN", l.tag, format, args...)
	}
}
func (l *slog) Warnf(format string, args ...interface{}) {
	lvl := l.Level()
	if lvl <= LevelWarn {
		l.b.printf("WRN", l.tag, format, args...)
	}
}
// Warnf formats message according to format specifier, prepends the prefix as
// necessary, and writes to log with LevelWarn.
//
// This is part of the Logger interface implementation.