Add HTTP middleware

[fproulx-dfuse]

It'd be nice to provide a simple HTTP middleware to avoid the boilerplate of adding the headers.

[sergolius]

@fproulx-eoscanada
middlewares:

func authorizeMiddleware(next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		ctx := context.WithValue(r.Context(), "user_id", "user-12345")
		next(w, r.WithContext(ctx))
	}
}

func limiterMiddleware(rateLimiter *redis_rate.Limiter, limit int64, next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		userID := r.Context().Value("user_id").(string)

		rate, delay, allowed := rateLimiter.AllowMinute(userID, limit)
		if !allowed {
			h := w.Header()
			h.Set("X-RateLimit-Limit", strconv.FormatInt(limit, 10))
			h.Set("X-RateLimit-Remaining", strconv.FormatInt(limit-rate, 10))
			delaySec := int64(delay / time.Second)
			h.Set("X-RateLimit-Delay", strconv.FormatInt(delaySec, 10))
			http.Error(w, "API rate limit exceeded.", 429)

			buf := new(bytes.Buffer)
			buf.WriteByte('\n')
			fmt.Fprintf(buf, "Current rate: %d\n", rate)
			fmt.Fprintf(buf, "Delay: %s\n", delay)
			fmt.Fprintf(buf, "Allowed: %v\n", allowed)
			fmt.Fprintf(buf, "Rate limit remaining: %s\n", strconv.FormatInt(limit-rate, 10))
			buf.WriteTo(w)
			return
		}

		next(w, r)
	}
}

func handler(w http.ResponseWriter, _ *http.Request) {
	fmt.Fprintf(w, "Hello world!\n")
}

func main:

redisClient := redis.NewClient(...)
limiter := redis_rate.NewLimiter(redisClient)
limit := int64(5)
http.HandleFunc("/", authorizeMiddleware(limiterMiddleware(limiter, limit, handler)))
log.Fatal(http.ListenAndServe("localhost:8820", nil))