Remove unnecessary generic type arguments

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175155365

java/google/registry/monitoring/metrics/MetricReporter.java

@@ -117,7 +117,7 @@ public class MetricReporter extends AbstractScheduledService {
     runOneIteration();
 
     // Offer a poision pill to inform the exporter to stop.
-    writeQueue.offer(Optional.<ImmutableList<MetricPoint<?>>>empty());
+    writeQueue.offer(Optional.empty());
     try {
       metricExporter.awaitTerminated(10, TimeUnit.SECONDS);
       logger.info("Shut down MetricExporter");