Add App Engine Datastore stats sample.

Stats queries aren't supported on the local server for unit testing, so
no tests. :-(

Sample from https://cloud.google.com/appengine/docs/java/datastore/stats

Author: tswast

appengine/datastore/src/main/java/com/example/appengine/StatsServlet.java

@@ -0,0 +1,49 @@
+/*
+ * 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.
+ */
+
+package com.example.appengine;
+
+import com.google.appengine.api.datastore.DatastoreService;
+import com.google.appengine.api.datastore.DatastoreServiceFactory;
+import com.google.appengine.api.datastore.Entity;
+import com.google.appengine.api.datastore.Query;
+
+import java.io.IOException;
+import java.io.PrintWriter;
+
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+public class StatsServlet extends HttpServlet {
+  @Override
+  protected void doGet(HttpServletRequest req, HttpServletResponse resp)
+      throws ServletException, IOException {
+    // [START stat_example]
+    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
+    Entity globalStat = datastore.prepare(new Query("__Stat_Total__")).asSingleEntity();
+    Long totalBytes = (Long) globalStat.getProperty("bytes");
+    Long totalEntities = (Long) globalStat.getProperty("count");
+    // [END stat_example]
+
+    resp.setContentType("text/plain");
+    resp.setCharacterEncoding("UTF-8");
+    PrintWriter w = resp.getWriter();
+    w.printf("%d bytes\n%d entities\n", totalBytes, totalEntities);
+  }
+}
+// [END cursors]

appengine/datastore/src/main/webapp/WEB-INF/web.xml

@@ -50,6 +50,14 @@
     <servlet-name>projection</servlet-name>
     <url-pattern>/projection</url-pattern>
   </servlet-mapping>
+  <servlet>
+    <servlet-name>stats</servlet-name>
+    <servlet-class>com.example.appengine.StatsServlet</servlet-class>
+  </servlet>
+  <servlet-mapping>
+    <servlet-name>stats</servlet-name>
+    <url-pattern>/stats</url-pattern>
+  </servlet-mapping>
 
   <!-- Special handler to populate Datastore with default values. -->
   <servlet>
@@ -73,4 +81,17 @@
       <role-name>*</role-name> <!-- Require users to login. -->
     </auth-constraint>
   </security-constraint>
+
+  <security-constraint>
+    <web-resource-collection>
+      <web-resource-name>profile</web-resource-name>
+      <url-pattern>/stats</url-pattern>
+    </web-resource-collection>
+    <user-data-constraint>
+      <transport-guarantee>CONFIDENTIAL</transport-guarantee>
+    </user-data-constraint>
+    <auth-constraint>
+      <role-name>admin</role-name>
+    </auth-constraint>
+  </security-constraint>
 </web-app>