Start adding basic metrics for NoteDb operations

Change-Id: Id903aa01aa3201b002ac09756f4096934fcaccec

gerrit-server/src/main/java/com/google/gerrit/server/notedb/NoteDbTable.java

@@ -0,0 +1,28 @@
+// Copyright (C) 2016 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.server.notedb;
+
+enum NoteDbTable {
+  CHANGES;
+
+  String key() {
+    return name().toLowerCase();
+  }
+
+  @Override
+  public String toString() {
+    return key();
+  }
+}