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.

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

java/google/registry/monitoring/whitebox/WhiteboxModule.java

@@ -35,9 +35,9 @@ public class WhiteboxModule {
 
   @Provides
   @IntoMap
-  @StringKey(EppMetrics.TABLE_ID)
+  @StringKey(EppMetric.TABLE_ID)
   static ImmutableList<TableFieldSchema> provideEppMetricsSchema() {
-    return EppMetrics.SCHEMA_FIELDS;
+    return EppMetric.SCHEMA_FIELDS;
   }
 
   @Provides