format.go
func SetFormatter(f Formatter) {
	formatter.Lock()
	defer formatter.Unlock()
	formatter.def = f
}
func SetFormatter(f Formatter) {
	formatter.Lock()
	defer formatter.Unlock()
	formatter.def = f
}
// SetFormatter sets the default formatter for all new backends. A backend will
// fetch this value once it is needed to format a record. Note that backends
// will cache the formatter after the first point. For now, make sure to set
// the formatter before logging.