annotate interface methods with non null

Signed-off-by: Alfredo Gutierrez <[email protected]>

server/src/main/java/com/hedera/block/server/metrics/MetricsService.java

@@ -18,6 +18,7 @@
 
 import com.swirlds.metrics.api.Counter;
 import com.swirlds.metrics.api.LongGauge;
+import edu.umd.cs.findbugs.annotations.NonNull;
 
 /** Use member variables of this class to update metric data for the Hedera Block Node. */
 public interface MetricsService {
@@ -26,26 +27,30 @@ public interface MetricsService {
      *
      * @return use this metric to increase the counter of block items received
      */
+    @NonNull
     Counter liveBlockItems();
 
     /**
      * Update the counter of blocks persisted to storage.
      *
      * @return use this counter to increase the amount of blocks persisted to disk
      */
+    @NonNull
     Counter blocksPersisted();
 
     /**
      * Update the counter of single blocks retrieved from storage.
      *
      * @return use this metric to increase the counter of single blocks retrieved
      */
+    @NonNull
     Counter singleBlocksRetrieved();
 
     /**
      * Update the gauge of subscribers currently consuming to the live stream.
      *
      * @return Use this to increase or decrease the amount of current subscribers to the live stream
      */
+    @NonNull
     LongGauge subscribers();
 }