Add nomulus count_domains command

This efficiently counts domains without having to load them (as opposed to the
existing list_domains command which does load then).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194559095

java/google/registry/tools/CountDomainsCommand.java

@@ -0,0 +1,57 @@
+// 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.tools;
+
+import static google.registry.model.ofy.ObjectifyService.ofy;
+import static google.registry.model.registry.Registries.assertTldsExist;
+
+import com.beust.jcommander.Parameter;
+import com.beust.jcommander.Parameters;
+import com.google.common.collect.Iterators;
+import google.registry.model.domain.DomainResource;
+import google.registry.tools.Command.RemoteApiCommand;
+import google.registry.util.Clock;
+import java.util.List;
+import javax.inject.Inject;
+import org.joda.time.DateTime;
+
+/** Command to show the count of active domains on a given TLD. */
+@Parameters(separators = " =", commandDescription = "Show count of domains on TLD")
+final class CountDomainsCommand implements RemoteApiCommand {
+
+  @Parameter(description = "TLD(s) to count domains on", required = true)
+  private List<String> mainParameters;
+
+  @Inject Clock clock;
+
+  @Override
+  public void run() {
+    DateTime now = clock.nowUtc();
+    assertTldsExist(mainParameters)
+        .forEach(tld -> System.out.printf("%s,%d\n", tld, getCountForTld(tld, now)));
+  }
+
+  private int getCountForTld(String tld, DateTime now) {
+    return Iterators.size(
+        ofy()
+            .load()
+            .type(DomainResource.class)
+            .filter("tld", tld)
+            .filter("deletionTime >", now)
+            .chunkAll()
+            .keys()
+            .iterator());
+  }
+}