type logWriter struct {
}

func (writer logWriter) Write(bytes []byte) (int, error) {
    return fmt.Print(time.Now().UTC().Format("2006-01-02T15:04:05.999Z") + " [DEBUG] " + string(bytes))
}

func main() {

    log.SetFlags(0)
    log.SetOutput(new(logWriter))
    log.Println("This is something being logged!")
}

How to change the date/time format of Go’s log package - Stack Overflow