Add listener interface for usage data published events

Introduces a new event which allows to publish usage data of projects.
An example could be the sizes of the repositories in the file system.

Change-Id: I9a9773c3358f5b8a45db2ec8ba29b8a8ea6c3589
Signed-off-by: Stefan Lay <stefan.lay@sap.com>
Signed-off-by: Adrian Görler <adrian.goerler@sap.com>

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/events/UsageDataPublishedListener.java

@@ -0,0 +1,45 @@
+// Copyright (C) 2014 The Android Open Source Project
+//
+// 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.google.gerrit.extensions.events;
+
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+
+import java.sql.Timestamp;
+import java.util.List;
+
+/** Notified when usage data is published */
+@ExtensionPoint
+public interface UsageDataPublishedListener {
+
+  public interface Event {
+    MetaData getMetaData();
+    Timestamp getInstant();
+    List<Data> getData();
+  }
+
+  public interface Data {
+    long getValue();
+    String getProjectName();
+  }
+
+  public interface MetaData {
+    public String getName();
+    public String getUnitName();
+    public String getUnitSymbol();
+    public String getDescription();
+  }
+
+  void onUsageDataPublished(Event event);
+}