You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: logger.go
    path of file: ./repos/req
    
    the code of the file until where you have to start completion: package req

import (
	"io"
	"log"
	"os"
)

// Logger is the abstract logging interface, gives control to
// the Req user
    
func (l *logger) output(level, format string, v ...interface{}) {
	format = level + " [req] " + format
	if len(v) == 0 {
		l.l.Print(format)
		return
	}
	l.l.Printf(format, v...)
}