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/MetricSchema.java

@@ -0,0 +1,71 @@
+// 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.common.collect.ImmutableSet;
+
+/** The description of a metric's schema. */
+@AutoValue
+public abstract class MetricSchema {
+
+  MetricSchema() {}
+
+  /**
+   * Returns an instance of {@link MetricSchema}.
+   *
+   * @param name must have a URL-like hierarchical name, for example "/cpu/utilization".
+   * @param description a human readable description of the metric. Must not be blank.
+   * @param valueDisplayName a human readable description of the metric's value. Must not be blank.
+   * @param kind the kind of metric.
+   * @param labels an ordered set of mandatory metric labels. For example, a metric may track error
+   *     code as a label. If labels are set, corresponding label values must be provided when values
+   *     are set. The set of labels may be empty.
+   */
+  static MetricSchema create(
+      String name,
+      String description,
+      String valueDisplayName,
+      Kind kind,
+      ImmutableSet<LabelDescriptor> labels) {
+    checkArgument(!name.isEmpty(), "Name must not be blank");
+    checkArgument(!description.isEmpty(), "Description must not be blank");
+    checkArgument(!valueDisplayName.isEmpty(), "Value Display Name must not be empty");
+    // TODO: strengthen metric name validation.
+
+    return new AutoValue_MetricSchema(name, description, valueDisplayName, kind, labels);
+  }
+
+  public abstract String name();
+
+  public abstract String description();
+
+  public abstract String valueDisplayName();
+
+  public abstract Kind kind();
+
+  public abstract ImmutableSet<LabelDescriptor> labels();
+
+  /**
+   * The kind of metric. CUMULATIVE metrics have values relative to an initial value, and GAUGE
+   * metrics have values which are standalone.
+   */
+  public enum Kind {
+    CUMULATIVE,
+    GAUGE,
+  }
+}