Add a Nomulus command to list Datastore operations

Command would be useful when checking the status of daily
backup jobs.

Only two modes are supported: list all operations, or list those
that started within the past period as specified by command line
option.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=229239783

java/google/registry/tools/ListDatastoreOperationsCommand.java

@@ -0,0 +1,63 @@
+// Copyright 2019 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.tools;
+
+import com.beust.jcommander.Parameter;
+import com.beust.jcommander.Parameters;
+import com.google.api.client.json.JsonFactory;
+import com.google.api.client.json.jackson2.JacksonFactory;
+import google.registry.export.datastore.DatastoreAdmin;
+import google.registry.export.datastore.DatastoreAdmin.ListOperations;
+import google.registry.util.Clock;
+import java.util.Optional;
+import javax.annotation.Nullable;
+import javax.inject.Inject;
+import org.joda.time.DateTime;
+import org.joda.time.Duration;
+
+/** Command that lists Datastore operations. */
+@Parameters(separators = " =", commandDescription = "List Datastore operations.")
+public class ListDatastoreOperationsCommand implements Command {
+
+  private static final JsonFactory JSON_FACTORY = JacksonFactory.getDefaultInstance();
+
+  @Nullable
+  @Parameter(
+      names = "--start_time_filter",
+      description =
+          "Duration relative to current time, used to filter operations by start time. "
+              + "Value is in ISO-8601 format, e.g., PT10S for 10 seconds.")
+  private Duration startTimeFilter;
+
+  @Inject DatastoreAdmin datastoreAdmin;
+  @Inject Clock clock;
+
+  @Override
+  public void run() throws Exception {
+    ListOperations listOperations =
+        getQueryFilter().map(datastoreAdmin::list).orElseGet(() -> datastoreAdmin.listAll());
+    System.out.println(JSON_FACTORY.toPrettyString(listOperations.execute()));
+  }
+
+  private Optional<String> getQueryFilter() {
+    if (startTimeFilter == null) {
+      return Optional.empty();
+    }
+
+    DateTime earliestStartingTime = clock.nowUtc().minus(startTimeFilter);
+    return Optional.of(
+        String.format("metadata.common.startTime>\"%s\"", earliestStartingTime));
+  }
+}