Add golang echo backend sample (envoyproxy#35)

* Add golang echo backend sample

Add sample Go backend server so that nodejs version and dependency can
be removed. Go is likely already present on build system due to other
istio components usage.

* Remove nodejs echo backend

src/envoy/prototype/README.md

@@ -33,8 +33,7 @@ This Proxy will use Envoy and talk to Mixer server.
 
 ```
   cd test/backend/echo
-  npm install
-  node echo.js
+  go run echo.go
 ```
 
 * Start Envoy proxy, run

test/backend/echo/README.md

@@ -1,3 +1,3 @@
-# Simple echo server for esp testing.
+# Simple echo server for proxy testing.
 
-Node.js implementation of echo server.
+golang implementation of echo server.

test/backend/echo/echo.go

@@ -0,0 +1,76 @@
+// Copyright 2016 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+// An example implementation of Echo backend in go.
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"strconv"
+	"sync"
+	"time"
+)
+
+var (
+	port = flag.Int("port", 8080, "default http port")
+
+	mu       sync.Mutex
+	requests = 0
+	data     = 0
+)
+
+func handler(w http.ResponseWriter, r *http.Request) {
+	body, err := ioutil.ReadAll(r.Body)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	// echo back the Content-Type and Content-Length in the response
+	for _, k := range []string{"Content-Type", "Content-Length"} {
+		if v := r.Header.Get(k); v != "" {
+			w.Header().Set(k, v)
+		}
+	}
+	w.WriteHeader(http.StatusOK)
+	w.Write(body)
+
+	mu.Lock()
+	requests++
+	data += len(body)
+	defer mu.Unlock()
+}
+
+func main() {
+	flag.Parse()
+
+	go func() {
+		for {
+			mu.Lock()
+			fmt.Printf("Requests Requests: %v  Data: %v\n", requests, data)
+			mu.Unlock()
+			time.Sleep(time.Second)
+		}
+	}()
+
+	fmt.Printf("Listening on port %v\n", *port)
+
+	http.HandleFunc("/", handler)
+	http.ListenAndServe(":"+strconv.Itoa(*port), nil)
+}