Add singleton scope to KafkaSubscriberProperties

Change-Id: If1df5aaa491681e381835703b854567e79d0187d
diff --git a/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaSubscriberProperties.java b/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaSubscriberProperties.java
index c3fd917..d2ba805 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaSubscriberProperties.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/kafka/config/KafkaSubscriberProperties.java
@@ -17,7 +17,9 @@
 import com.google.gerrit.extensions.annotations.PluginName;
 import com.google.gerrit.server.config.PluginConfigFactory;
 import com.google.inject.Inject;
+import com.google.inject.Singleton;
 
+@Singleton
 public class KafkaSubscriberProperties extends KafkaProperties {
   private static final long serialVersionUID = 1L;
   private static final String DEFAULT_POLLING_INTERVAL_MS = "1000";