Fix visibility on MetricsReporter#startMetricExporter()

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130576851

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

@@ -161,8 +161,7 @@ public class MetricReporter extends AbstractScheduledService {
     return executor;
   }
 
-  @VisibleForTesting
-  void startMetricExporter() {
+  private void startMetricExporter() {
     // Services in the FAILED state must be reconstructed, they can't be started
     if (metricExporter.state() == State.FAILED) {
       logger.log(