Clean up misc Javadoc and deprecation warnings

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136156922

java/google/registry/monitoring/metrics/MetricMetrics.java

@@ -49,6 +49,7 @@ final class MetricMetrics {
               LABELS);
 
   /** A gauge representing a snapshot of the number of active timeseries being reported. */
+  @SuppressWarnings("unused")
   private static final Metric<Long> timeseriesCount =
       MetricRegistryImpl.getDefault()
           .newGauge(