An example of turning https://github.com/go-kit/kit/tree/master/log into usable HTTP middleware for Go.

logger.go

package handler

import (
	"bufio"
	"io"
	"net"
	"net/http"
	"time"

	"github.com/go-kit/kit/log"
)

type logger struct {
	handler http.Handler
	output  log.Logger
	// opts    options
}

// NewLogger returns a handler for logging the HTTP request, including status
// code, HTTP method, URI (path), duration, bytes written and client address.
func NewLogger(output log.Logger) func(http.Handler) http.Handler {
	return func(h http.Handler) http.Handler {
		lgr := &logger{
			handler: h,
			output:  output,
		}

		return lgr
	}
}

func (lg *logger) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	ww := wrapWriter(w)

	start := time.Now()
	lg.handler.ServeHTTP(ww, r)
	end := time.Now()
	duration := end.Sub(start)

	lg.output.Log(
		"ts", time.Now().Format(time.RFC3339),
		"status", ww.Status(),
		"method", r.Method,
		"uri", r.URL.Path,
		"duration", duration,
		"bytes", ww.BytesWritten(),
		"from", r.RemoteAddr,
	)
}

// The below writer-wrapping code has been lifted from
// https://github.com/zenazn/goji/blob/master/web/middleware/logger.go - because
// it does exactly what is needed, and it's unlikely to change in any
// significant way that makes copying worse-off than importing. MIT licensed
// and (c) Carl Jackson.

// writerProxy is a proxy around an http.ResponseWriter that allows you to hook
// into various parts of the response process.
type writerProxy interface {
	http.ResponseWriter
	// Status returns the HTTP status of the request, or 0 if one has not
	// yet been sent.
	Status() int
	// BytesWritten returns the total number of bytes sent to the client.
	BytesWritten() int
	// Tee causes the response body to be written to the given io.Writer in
	// addition to proxying the writes through. Only one io.Writer can be
	// tee'd to at once: setting a second one will overwrite the first.
	// Writes will be sent to the proxy before being written to this
	// io.Writer. It is illegal for the tee'd writer to be modified
	// concurrently with writes.
	Tee(io.Writer)
	// Unwrap returns the original proxied target.
	Unwrap() http.ResponseWriter
}

// wrapWriter wraps an http.ResponseWriter, returning a proxy that allows you to
// hook into various parts of the response process.
func wrapWriter(w http.ResponseWriter) writerProxy {
	_, cn := w.(http.CloseNotifier)
	_, fl := w.(http.Flusher)
	_, hj := w.(http.Hijacker)
	_, rf := w.(io.ReaderFrom)

	bw := basicWriter{ResponseWriter: w}
	if cn && fl && hj && rf {
		return &fullWriter{bw}
	}
	if fl {
		return &flushWriter{bw}
	}
	return &bw
}

// basicWriter wraps a http.ResponseWriter that implements the minimal
// http.ResponseWriter interface.
type basicWriter struct {
	http.ResponseWriter
	wroteHeader bool
	code        int
	bytes       int
	tee         io.Writer
}

func (b *basicWriter) WriteHeader(code int) {
	if !b.wroteHeader {
		b.code = code
		b.wroteHeader = true
		b.ResponseWriter.WriteHeader(code)
	}
}
func (b *basicWriter) Write(buf []byte) (int, error) {
	b.WriteHeader(http.StatusOK)
	n, err := b.ResponseWriter.Write(buf)
	if b.tee != nil {
		_, err2 := b.tee.Write(buf[:n])
		// Prefer errors generated by the proxied writer.
		if err == nil {
			err = err2
		}
	}
	b.bytes += n
	return n, err
}
func (b *basicWriter) maybeWriteHeader() {
	if !b.wroteHeader {
		b.WriteHeader(http.StatusOK)
	}
}
func (b *basicWriter) Status() int {
	return b.code
}
func (b *basicWriter) BytesWritten() int {
	return b.bytes
}
func (b *basicWriter) Tee(w io.Writer) {
	b.tee = w
}
func (b *basicWriter) Unwrap() http.ResponseWriter {
	return b.ResponseWriter
}

// fullWriter is a writer that additionally satisfies http.CloseNotifier,
// http.Flusher, http.Hijacker, and io.ReaderFrom. It exists for the common case
// of wrapping the http.ResponseWriter that package http gives you, in order to
// make the proxied object support the full method set of the proxied object.
type fullWriter struct {
	basicWriter
}

func (f *fullWriter) CloseNotify() <-chan bool {
	cn := f.basicWriter.ResponseWriter.(http.CloseNotifier)
	return cn.CloseNotify()
}
func (f *fullWriter) Flush() {
	fl := f.basicWriter.ResponseWriter.(http.Flusher)
	fl.Flush()
}
func (f *fullWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	hj := f.basicWriter.ResponseWriter.(http.Hijacker)
	return hj.Hijack()
}
func (f *fullWriter) ReadFrom(r io.Reader) (int64, error) {
	if f.basicWriter.tee != nil {
		return io.Copy(&f.basicWriter, r)
	}
	rf := f.basicWriter.ResponseWriter.(io.ReaderFrom)
	f.basicWriter.maybeWriteHeader()
	return rf.ReadFrom(r)
}

var _ http.CloseNotifier = &fullWriter{}
var _ http.Flusher = &fullWriter{}
var _ http.Hijacker = &fullWriter{}
var _ io.ReaderFrom = &fullWriter{}

type flushWriter struct {
	basicWriter
}

func (f *flushWriter) Flush() {
	fl := f.basicWriter.ResponseWriter.(http.Flusher)
	fl.Flush()
}

var _ http.Flusher = &flushWriter{}