Add RDAP metric class

The code to use this class will come in future CLs, to avoid a huge CL.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175162557

java/google/registry/rdap/RdapNameserverSearchAction.java

@@ -212,7 +212,7 @@ public class RdapNameserverSearchAction extends RdapActionBase {
         }
       }
     }
-    return makeSearchResults(hostList, IncompletenessWarningType.NONE, now);
+    return makeSearchResults(hostList, IncompletenessWarningType.COMPLETE, now);
   }
 
   /**