Add StackDriver implementation, in monitoring/metrics package

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130287319

java/google/registry/monitoring/metrics/LabelDescriptor.java

@@ -0,0 +1,56 @@
+// Copyright 2016 The Domain Registry 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.monitoring.metrics;
+
+import static com.google.common.base.Preconditions.checkArgument;
+
+import com.google.auto.value.AutoValue;
+import com.google.re2j.Pattern;
+
+/**
+ * Definition of a metric label.
+ *
+ * <p>If a metric is created with labels, corresponding label values must be provided when setting
+ * values on the metric.
+ */
+@AutoValue
+public abstract class LabelDescriptor {
+
+  private static final Pattern ALLOWED_LABEL_PATTERN = Pattern.compile("\\w+");
+
+  LabelDescriptor() {}
+
+  /**
+   * Returns a new {@link LabelDescriptor}.
+   *
+   * @param name identifier for label
+   * @param description human-readable description of label
+   * @throws IllegalArgumentException if {@code name} isn't {@code \w+} or {@code description} is
+   *     blank
+   */
+  public static LabelDescriptor create(String name, String description) {
+    checkArgument(
+        ALLOWED_LABEL_PATTERN.matches(name),
+        "Label must match the regex %s",
+        ALLOWED_LABEL_PATTERN);
+    checkArgument(!description.isEmpty(), "Description must not be empty");
+
+    return new AutoValue_LabelDescriptor(name, description);
+  }
+
+  public abstract String name();
+
+  public abstract String description();
+}