Add recipe to illustrated middleware

recipes/middleware/.gitignore

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+/middleware

recipes/middleware/main.go

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+// Copyright 2012-present Oliver Eilhard. All rights reserved.
+// Use of this source code is governed by a MIT-license.
+// See http://olivere.mit-license.org/license.txt for details.
+
+// Middleware via HTTP RoundTripper.
+//
+// Example
+//
+//     middleware "http://127.0.0.1:9200/test-index?sniff=false&healthcheck=false"
+//
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"net/http"
+	"sync/atomic"
+
+	"github.com/olivere/elastic/v7"
+)
+
+// CountingTransport will count requests.
+type CountingTransport struct {
+	N    int64             // number of requests passing this transport
+	next http.RoundTripper // next round-tripper or http.DefaultTransport if nil
+}
+
+// RoundTrip implements a transport that will count requests.
+func (tr *CountingTransport) RoundTrip(r *http.Request) (*http.Response, error) {
+	atomic.AddInt64(&tr.N, 1)
+	if tr.next != nil {
+		return tr.next.RoundTrip(r)
+	}
+	return http.DefaultTransport.RoundTrip(r)
+}
+
+func main() {
+	var (
+		url   = flag.String("url", "http://localhost:9200", "Elasticsearch URL")
+		sniff = flag.Bool("sniff", true, "Enable or disable sniffing")
+	)
+	flag.Parse()
+	log.SetFlags(0)
+
+	if *url == "" {
+		*url = "http://127.0.0.1:9200"
+	}
+
+	tr := &CountingTransport{}
+
+	// Create an Elasticsearch client
+	client, err := elastic.NewClient(
+		elastic.SetURL(*url),
+		elastic.SetSniff(*sniff),
+		elastic.SetHttpClient(&http.Client{
+			Transport: tr,
+		}),
+	)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Get ES version
+	indices, err := client.IndexNames()
+	if err != nil {
+		log.Fatal(err)
+	}
+	for _, index := range indices {
+		fmt.Println(index)
+	}
+
+	// Just a status message
+	fmt.Printf("%d requests executed.\n", tr.N)
+}