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

javatests/google/registry/monitoring/metrics/example/SheepCounterExample.java

@@ -46,7 +46,7 @@ import java.util.logging.Logger;
 /** A sample application which uses the Metrics API to count sheep while sleeping. */
 public final class SheepCounterExample {
 
-  /**
+  /*
    * The code below for using a custom {@link LogManager} is only necessary to enable logging at JVM
    * shutdown to show the shutdown logs of {@link MetricReporter} in this small standalone
    * application.