code: func StdLogObserver(l *log.Logger) Observer {
  	const (
  		logFmt  = "time=%q status=%d method=%q uri=%q size_bytes=%d duration_ms=%0.02f"
  		dateFmt = "2006-01-02T15:04:05.9999"
  	)
  	return func(result Result) {
  		l.Printf(
  			logFmt,
  			time.Now().Format(dateFmt),
  			result.Status,
  			result.Method,
  			result.URI,
  			result.Size,
  			result.Duration.Seconds()*1e3, // https://github.com/golang/go/issues/5491#issuecomment-66079585
  		)
  	}
  }
docs: // StdLogObserver creates an Observer that will log each request in structured
  // format using the given stdlib logger