Fix warning

src/main/java/com/googlesource/gerrit/plugins/metricsreporters/GerritPrometheusExporter.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/metricsreporters/GerritPrometheusExporter.java b/src/main/java/com/googlesource/gerrit/plugins/metricsreporters/GerritPrometheusExporter.java
index 54827be..894a674 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/metricsreporters/GerritPrometheusExporter.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/metricsreporters/GerritPrometheusExporter.java
@@ -25,6 +25,7 @@
 import javax.servlet.ServletResponse;
 import javax.servlet.http.HttpServletResponse;
 
+@SuppressWarnings("serial")
 @Singleton
 public class GerritPrometheusExporter extends MetricsServlet {
   CapabilityChecker capabilityChecker;