Backup Datastore using the Admin REST API

Add server end points to backup Datastore using managed-export mechanism.
A cron job is defined in Alpha to run daily exports using this implementation.

Existing backup is left running. The new backups are saved to a new set of
locations:
- GCS bucket: gs://PROJECT-ID-datastore-backups
- Big Query data set: datastore_backups
- Big Query latest back up view name: latest_datastore_backup
Also, the names of Bigquery tables now use the export timestamp
assigned by Datastore. E.g., 2018_12_05T23_56_18_50532_ContactResource,

After the new import mechanism is implemented and the back-restore flow is
tested, we will stop the existing backup runs and deploy the new
implementation to all environments.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=224932957

java/google/registry/export/BackupDatastoreAction.java

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+// Copyright 2018 The Nomulus Authors. 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 google.registry.export;
+
+import static google.registry.export.CheckBackupAction.enqueuePollTask;
+import static google.registry.request.Action.Method.POST;
+
+import com.google.common.flogger.FluentLogger;
+import google.registry.config.RegistryConfig;
+import google.registry.export.datastore.DatastoreAdmin;
+import google.registry.export.datastore.Operation;
+import google.registry.request.Action;
+import google.registry.request.HttpException.InternalServerErrorException;
+import google.registry.request.Response;
+import google.registry.request.auth.Auth;
+import javax.inject.Inject;
+
+/**
+ * Action to trigger a Datastore backup job that writes a snapshot to Google Cloud Storage. This
+ * class is introduced as an experimental feature, and will eventually replace {@link
+ * ExportSnapshotAction}.
+ *
+ * <p>This is the first step of a four step workflow for exporting snapshots, with each step calling
+ * the next upon successful completion:
+ *
+ * <ol>
+ *   <li>The snapshot is exported to Google Cloud Storage (this action).
+ *   <li>The {@link CheckBackupAction} polls until the export is completed.
+ *   <li>The {@link UploadDatastoreBackupAction} uploads the data from GCS to BigQuery.
+ *   <li>The {@link UpdateSnapshotViewAction} updates the view in latest_datastore_export.
+ * </ol>
+ */
+@Action(
+    path = BackupDatastoreAction.PATH,
+    method = POST,
+    automaticallyPrintOk = true,
+    auth = Auth.AUTH_INTERNAL_ONLY)
+public class BackupDatastoreAction implements Runnable {
+
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+
+  /** Queue to use for enqueuing the task that will actually launch the backup. */
+  static final String QUEUE = "export-snapshot"; // See queue.xml.
+
+  static final String PATH = "/_dr/task/backupDatastore"; // See web.xml.
+
+  @Inject DatastoreAdmin datastoreAdmin;
+  @Inject Response response;
+
+  @Inject
+  BackupDatastoreAction() {}
+
+  @Override
+  public void run() {
+    try {
+      Operation backup =
+          datastoreAdmin
+              .export(RegistryConfig.getDatastoreBackupsBucket(), ExportConstants.getBackupKinds())
+              .execute();
+
+      String backupName = backup.getName();
+      // Enqueue a poll task to monitor the backup and load REPORTING-related kinds into bigquery.
+      enqueuePollTask(backupName, ExportConstants.getReportingKinds());
+      String message =
+          String.format(
+              "Datastore backup started with name: %s\nSaving to %s",
+              backupName, backup.getExportFolderUrl());
+      logger.atInfo().log(message);
+      response.setPayload(message);
+    } catch (Throwable e) {
+      throw new InternalServerErrorException("Exception occurred while backing up datastore.", e);
+    }
+  }
+}