docs: add a sample app using OAuth2 authorization flow

samples/quickstart_oauth2/README.md

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+This application demonstrates the usage of the Analytics Data API using
+OAuth2 credentials.
+
+Please familiarize yourself with the OAuth2 flow guide at
+https://developers.google.com/identity/protocols/oauth2
+
+For more information on authenticating as an end user, see
+https://cloud.google.com/docs/authentication/end-user
+
+In a nutshell, you need to:
+
+1. Create your OAuth2 client credentials in Google Cloud Console.
+Choose "Web application" when asked for an application type.
+https://support.google.com/cloud/answer/6158849
+
+2. When configuring the web application credentials, add
+"http://localhost:3000" to "Authorized redirect URIs".
+
+3. Download a credentials file using "Download JSON" button in the credentials
+configuration dialog and save it as `oauth2.keys.json` in the same
+directory with this sample app.
+
+4. Uncomment `$property_id` variable below and specify the
+value of the GA4 property id you want to access.
+
+5. Run the following commands from the current directory in order to install
+dependencies and run the sample app:
+
+```
+composer install
+php -S localhost:3000 -t .
+```
+6. In a browser, open the following url to start the sample:
+
+https://localhost:3000/

samples/quickstart_oauth2/composer.json

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+{
+    "require": {
+        "google/analytics-data": "^0.1.0",
+        "google/auth": "^1.14",
+        "ext-bcmath":  "*"
+    }
+}

samples/quickstart_oauth2/index.php

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+<?php
+// Copyright 2021 Google LLC
+//
+// 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.
+
+$property_id = 'YOUR-GA4-PROPERTY-ID';
+// [START analytics_data_quickstart_oauth2]
+
+require 'vendor/autoload.php';
+
+use Google\Analytics\Data\V1alpha\AlphaAnalyticsDataClient;
+use Google\Analytics\Data\V1alpha\DateRange;
+use Google\Analytics\Data\V1alpha\Dimension;
+use Google\Analytics\Data\V1alpha\Entity;
+use Google\Analytics\Data\V1alpha\Metric;
+use Google\ApiCore\ApiException;
+use Google\Auth\OAuth2;
+
+/**
+ * TODO(developer): Uncomment this variable and replace with your GA4
+ *   property ID before running the sample.
+ */
+$property_id = '222596558';
+
+// Start a session to persist credentials.
+session_start();
+
+// Set authorization parameters.
+$s = file_get_contents('./oauth2.keys.json');
+$keys = json_decode($s);
+$oauth = new OAuth2([
+    'scope' => 'https://www.googleapis.com/auth/analytics.readonly',
+    'tokenCredentialUri' => 'https://oauth2.googleapis.com/token',
+    'authorizationUri' => $keys->{'web'}->{'auth_uri'},
+    'clientId' => $keys->{'web'}->{'client_id'},
+    'clientSecret' => $keys->{'web'}->{'client_secret'},
+    'redirectUri' => 'http://' . $_SERVER['HTTP_HOST'] . '/',
+]);
+
+if (isset($_SESSION['access_token']) && $_SESSION['access_token']) {
+    // This is the final step of the OAuth2 authorization process, where an
+    // OAuth2 access token is available and can be used to set up a client.
+    $oauth->updateToken(['refresh_token' => $_SESSION['access_token']]);
+
+    // Make an API call.
+    $client = new AlphaAnalyticsDataClient(['credentials' => $oauth]);
+    try {
+        $response = $client->runReport([
+            'entity' => new Entity([
+                'property_id' => $property_id
+            ]),
+            'dateRanges' => [
+                new DateRange([
+                    'start_date' => '2020-03-31',
+                    'end_date' => 'today',
+                ]),
+            ],
+            'dimensions' => [new Dimension(
+                [
+                    'name' => 'city',
+                ]),
+            ],
+            'metrics' => [new Metric(
+                [
+                    'name' => 'activeUsers',
+                ])
+            ]
+        ]);
+
+        // Print results of an API call.
+        print 'Report result: <br />';
+
+        foreach ($response->getRows() as $row) {
+            print $row->getDimensionValues()[0]->getValue()
+                . ' ' . $row->getMetricValues()[0]->getValue() . '<br />';
+        }
+    } catch (ApiException $e) {
+        // Print an error message.
+        print $e->getMessage();
+    }
+
+} elseif (isset($_GET['code']) && $_GET['code']) {
+    // If an OAuth2 authorization code is present in the URL, exchange it for
+    // an access token.
+    $oauth->setCode($_GET['code']);
+    $oauth->fetchAuthToken();
+
+    // Persist the acquired access token in a session.
+    $_SESSION['access_token'] = $oauth->getAccessToken();
+
+    // Refresh the current page.
+    $redirect_uri = 'http://' . $_SERVER['HTTP_HOST'] . '/';
+    header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));
+} else {
+    // Redirect to Google's OAuth 2.0 server.
+    $auth_url = $oauth->buildFullAuthorizationUri();
+    header('Location: ' . filter_var($auth_url, FILTER_SANITIZE_URL));
+}
+
+// [END analytics_data_quickstart_oauth2]