Refactor metrics servlet to be a Daggerized action

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=120235694

java/com/google/domain/registry/env/common/backend/WEB-INF/web.xml

@@ -13,13 +13,8 @@
     <load-on-startup>1</load-on-startup>
   </servlet>
 
-  <!-- Whitebox Metrics servlet. -->
-  <servlet>
-    <servlet-name>metrics</servlet-name>
-    <servlet-class>com.google.domain.registry.monitoring.whitebox.MetricsTaskServlet</servlet-class>
-  </servlet>
   <servlet-mapping>
-    <servlet-name>metrics</servlet-name>
+    <servlet-name>backend-servlet</servlet-name>
     <url-pattern>/_dr/task/metrics</url-pattern>
   </servlet-mapping>