Clean up some code quality issues

This removes some qualifiers that aren't necessary (e.g. public/abstract on interfaces, private on enum constructors, final on private methods, static on nested interfaces/enums), uses Java 8 lambdas and features where that's an improvement

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177182945

java/google/registry/monitoring/whitebox/EppMetric.java

@@ -110,9 +110,7 @@ public abstract class EppMetric implements BigQueryMetric {
    */
   private static <T> void addOptional(
       String key, Optional<T> value, ImmutableMap.Builder<String, String> map) {
-    if (value.isPresent()) {
-      map.put(key, value.get().toString());
-    }
+    value.ifPresent(t -> map.put(key, t.toString()));
   }
 
   /** Create an {@link EppMetric.Builder}. */