Refactor out handler file from server.go

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

gateway/handlers/forwarding_proxy.go

@@ -0,0 +1,49 @@
+package handlers
+
+import (
+	"log"
+	"net/http"
+	"net/http/httputil"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/alexellis/faas/gateway/metrics"
+	"github.com/alexellis/faas/gateway/types"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// MakeForwardingProxyHandler create a handler which forwards HTTP requests
+func MakeForwardingProxyHandler(proxy *httputil.ReverseProxy, metrics *metrics.MetricOptions) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		uri := r.URL.String()
+
+		log.Printf("> Forwarding [%s] to %s", r.Method, r.URL.String())
+		start := time.Now()
+
+		writeAdapter := types.NewWriteAdapter(w)
+		proxy.ServeHTTP(writeAdapter, r)
+
+		seconds := time.Since(start).Seconds()
+		log.Printf("< [%s] - %d took %f seconds\n", r.URL.String(), writeAdapter.GetHeaderCode(), seconds)
+
+		forward := "/function/"
+		if startsWith(uri, forward) {
+			log.Printf("function=%s", uri[len(forward):])
+
+			service := uri[len(forward):]
+
+			metrics.GatewayFunctionsHistogram.
+				WithLabelValues(service).
+				Observe(seconds)
+
+			code := strconv.Itoa(writeAdapter.GetHeaderCode())
+
+			metrics.GatewayFunctionInvocation.With(prometheus.Labels{"function_name": service, "code": code}).Inc()
+		}
+	}
+}
+
+func startsWith(value, token string) bool {
+	return len(value) > len(token) && strings.Index(value, token) == 0
+}