Add metrics for the new Check API

New metrics are necessary because the new API no longer wraps
an EPP flow, therefore does not get metrics for free.

Metrics include
 - An EventMetric for processing time
 - An IncrementableMetric for request count, with
   availability (available/reserved/registered) and
   pricing (standard/premium) fields

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=199708592

java/google/registry/flows/CheckApiMetrics.java

@@ -0,0 +1,70 @@
+// 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.flows;
+
+import static com.google.monitoring.metrics.EventMetric.DEFAULT_FITTER;
+
+import com.google.common.collect.ImmutableSet;
+import com.google.monitoring.metrics.EventMetric;
+import com.google.monitoring.metrics.IncrementableMetric;
+import com.google.monitoring.metrics.LabelDescriptor;
+import com.google.monitoring.metrics.MetricRegistryImpl;
+import google.registry.monitoring.whitebox.CheckApiMetric;
+import google.registry.monitoring.whitebox.CheckApiMetric.Availability;
+import google.registry.monitoring.whitebox.CheckApiMetric.Tier;
+import javax.inject.Inject;
+
+/** Helpers for updating domain check metrics. */
+public class CheckApiMetrics {
+
+  private static final ImmutableSet<LabelDescriptor> LABEL_DESCRIPTORS =
+      ImmutableSet.of(
+          LabelDescriptor.create("tier", "Price tier of the domain name."),
+          LabelDescriptor.create("availability", "Availability of the domain name."),
+          LabelDescriptor.create("status", "The return status of the check."));
+
+  private static final IncrementableMetric requests =
+      MetricRegistryImpl.getDefault()
+          .newIncrementableMetric(
+              "/check_api/requests", "Count of CheckApi Requests", "count", LABEL_DESCRIPTORS);
+
+  private static final EventMetric processingTime =
+      MetricRegistryImpl.getDefault()
+          .newEventMetric(
+              "/check_api/processing_time",
+              "CheckApi Processing Time",
+              "milliseconds",
+              LABEL_DESCRIPTORS,
+              DEFAULT_FITTER);
+
+  @Inject
+  CheckApiMetrics() {}
+
+  public void incrementCheckApiRequest(CheckApiMetric metric) {
+    requests.increment(
+        metric.tier().map(Tier::getDisplayLabel).orElse(""),
+        metric.availability().map(Availability::getDisplayLabel).orElse(""),
+        metric.status().getDisplayLabel());
+  }
+
+  public void recordProcessingTime(CheckApiMetric metric) {
+    long elapsedTime = metric.endTimestamp().getMillis() - metric.startTimestamp().getMillis();
+    processingTime.record(
+        elapsedTime,
+        metric.tier().map(Tier::getDisplayLabel).orElse(""),
+        metric.availability().map(Availability::getDisplayLabel).orElse(""),
+        metric.status().getDisplayLabel());
+  }
+}