add react single page app handler (proxy to dev server when $REACT_ENDPOINT is defined

fix example

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>

example/example.go

@@ -120,7 +120,7 @@ func run(opts ...service.Option) {
 		panic(err)
 	}
 	RegisterGreeterServer(svc, &GreeterHandler{})
-	metrics2.Register(svc)
+	metrics.Register(svc)
 	go func() {
 		if err := svc.Start(); err != nil {
 			panic(err)

react/ui.go

@@ -0,0 +1,55 @@
+// Copyright 2022 Linka Cloud  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.
+
+package react
+
+import (
+	"embed"
+	"io/fs"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"os"
+	"strings"
+)
+
+func NewHandler(dir embed.FS, subpath string) (http.Handler, error) {
+	if e := os.Getenv("REACT_ENDPOINT"); e != "" {
+		return newProxy(e)
+	}
+	return newStatic(dir, subpath)
+}
+
+func newStatic(dir embed.FS, subpath string) (http.Handler, error) {
+	s, err := fs.Sub(dir, subpath)
+	if err != nil {
+		return nil, err
+	}
+	fsrv := http.FileServer(http.FS(s))
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if _, err := fs.Stat(s, strings.TrimPrefix(r.URL.Path, "/")); err != nil {
+			r.URL.Path = "/"
+		}
+		fsrv.ServeHTTP(w, r)
+	}), nil
+}
+
+func newProxy(endpoint string) (http.Handler, error) {
+	u, err := url.Parse(endpoint)
+	if err != nil {
+		return nil, err
+	}
+	p := httputil.NewSingleHostReverseProxy(u)
+	return p, nil
+}