Refactor EppMetrics into the EppMetric value type

This change refactors EppMetrics from the mutable self-exporting thing that it
was into a real value type EppMetric, and delegates exporting functionality to the
BigQueryMetricsEnqueuer.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132387660

java/google/registry/monitoring/whitebox/BigQueryMetric.java

@@ -0,0 +1,36 @@
+// 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.whitebox;
+
+import com.google.api.services.bigquery.model.TableFieldSchema;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+
+/**
+ * A metric which can be encoded into a BigQuery row.
+ *
+ * @see BigQueryMetricsEnqueuer
+ */
+public interface BigQueryMetric {
+
+  /** Get the BigQuery table name for this metric. */
+  String getTableId();
+
+  /** Get the schema description for the BigQuery table. */
+  ImmutableList<TableFieldSchema> getSchemaFields();
+
+  /** Get a map of the row values for this metric instance. */
+  ImmutableMap<String, String> getBigQueryRowEncoding();
+}