adds flex metadata example (GoogleCloudPlatform#309)

Author: bshaffer

appengine/flexible/metadata/README.md

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+# Compute Metadata on App Engine Flexible Environment
+
+This sample application demonstrates how to access
+[Compute Metadata](https://cloud.google.com/compute/docs/storing-retrieving-metadata)
+from App Engine flexible environment.
+
+## Setup
+
+Before running this sample:
+
+### Register your application
+
+- Go to
+  [Google Developers Console](https://console.developers.google.com/project)
+  and create a new project. 
+
+### Prerequisites
+
+- Install [`composer`](https://getcomposer.org)
+- Install dependencies by running:
+
+    ```sh
+    composer install
+    ```
+
+- Install the [Google Cloud SDK](https://developers.google.com/cloud/sdk/).
+- Initialize the SDK by running `gcloud init`
+
+## Run Locally
+
+This sample is designed to run in App Engine flexible environment.
+This application will fail to reach the Metadata server if run locally.
+
+## Deploy to App Engine
+
+**Deploy with gcloud**
+
+```
+gcloud config set project YOUR_PROJECT_ID
+gcloud app deploy
+gcloud app browse
+```
+
+The last command will open `https://{YOUR_PROJECT_ID}.appspot.com/`
+in your browser.

appengine/flexible/metadata/app.php

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+
+/**
+ * Copyright 2017 Google Inc.
+ *
+ * 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.
+ */
+# [START app]
+use Silex\Application;
+
+# [START metadata]
+function get_external_ip_using_google_cloud()
+{
+    $metadata = new Google\Cloud\Compute\Metadata();
+    $externalIp = $metadata->get(
+        'instance/network-interfaces/0/access-configs/0/external-ip');
+
+    return $externalIp;
+}
+
+function get_external_ip_using_curl()
+{
+    $url = 'http://metadata.google.internal/computeMetadata/v1/' .
+        'instance/network-interfaces/0/access-configs/0/external-ip';
+
+    $ch = curl_init();
+    curl_setopt($ch, CURLOPT_URL, $url);
+    curl_setopt($ch, CURLOPT_HTTPHEADER, array('Metadata-Flavor: Google'));
+    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
+    return curl_exec($ch);
+}
+# [END metadata]
+
+// create the Silex application
+$app = new Application();
+
+$app->get('/', function () use ($app) {
+    if (!$externalIp = get_external_ip_using_google_cloud()) {
+        return 'Unable to reach Metadata server - are you running locally?';
+    }
+    return sprintf('External IP: %s', $externalIp);
+});
+
+$app->get('/curl', function () use ($app) {
+    if (!$externalIp = get_external_ip_using_curl()) {
+        return 'Unable to reach Metadata server - are you running locally?';
+    }
+    return sprintf('External IP: %s', $externalIp);
+});
+# [END app]
+
+return $app;

appengine/flexible/metadata/app.yaml

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+runtime: php
+env: flex

appengine/flexible/metadata/composer.json

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+{
+    "require": {
+        "silex/silex": "^1.3",
+        "google/cloud": "^0.20"
+    },
+    "require-dev": {
+        "symfony/browser-kit": "^3.0",
+        "google/cloud-tools":"^0.6"
+    }
+}