Make metrics export run on backend module

This fixes a previous issue where metrics generated on the
tools/backend modules weren't being successfully exported to BigQuery.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119047757

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

@@ -13,6 +13,16 @@
     <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>
+    <url-pattern>/_dr/task/metrics</url-pattern>
+  </servlet-mapping>
+
   <!-- RDE -->
 
   <!--

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

@@ -103,16 +103,6 @@
     <url-pattern>/check</url-pattern>
   </servlet-mapping>
 
-  <!-- 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>
-    <url-pattern>/_dr/task/metrics</url-pattern>
-  </servlet-mapping>
-
   <!-- Security config -->
   <security-constraint>
     <web-resource-collection>

java/com/google/domain/registry/monitoring/whitebox/Metrics.java

@@ -18,6 +18,8 @@ import static com.google.appengine.api.taskqueue.QueueFactory.getQueue;
 import static com.google.appengine.api.taskqueue.TaskOptions.Builder.withUrl;
 import static com.google.domain.registry.bigquery.BigqueryUtils.toBigqueryTimestamp;
 
+import com.google.appengine.api.modules.ModulesService;
+import com.google.appengine.api.modules.ModulesServiceFactory;
 import com.google.appengine.api.taskqueue.TaskOptions;
 import com.google.appengine.api.taskqueue.TransientFailureException;
 import com.google.common.base.Supplier;
@@ -33,12 +35,15 @@ import java.util.UUID;
 import java.util.concurrent.TimeUnit;
 
 /** A collector of metric information. */
-abstract class Metrics {
+public abstract class Metrics {
 
   private static final FormattingLogger logger = FormattingLogger.getLoggerForCallerClass();
 
   public static final String QUEUE = "bigquery-streaming-metrics";
 
+  @NonFinalForTesting
+  private static ModulesService modulesService = ModulesServiceFactory.getModulesService();
+
   @NonFinalForTesting
   private static Clock clock = new SystemClock();
 
@@ -60,7 +65,9 @@ abstract class Metrics {
 
   public void export() {
     try {
-      TaskOptions opts = withUrl("/_dr/task/metrics")
+      String hostname = modulesService.getVersionHostname("backend", null);
+      TaskOptions opts = withUrl(MetricsTaskServlet.PATH)
+          .header("Host", hostname)
           .param("insertId", idGenerator.get())
           .param("startTime", toBigqueryTimestamp(startTimeMillis, TimeUnit.MILLISECONDS))
           .param("endTime", toBigqueryTimestamp(clock.nowUtc().getMillis(), TimeUnit.MILLISECONDS));

java/com/google/domain/registry/monitoring/whitebox/MetricsTaskServlet.java

@@ -51,6 +51,8 @@ import javax.servlet.http.HttpServletResponse;
 /** Servlet for exporting metrics to BigQuery. */
 public class MetricsTaskServlet extends HttpServlet {
 
+  public static final String PATH = "/_dr/task/metrics";
+
   private static final FormattingLogger logger = FormattingLogger.getLoggerForCallerClass();
   private static final String DATASET_ID = "metrics";
   private static final String PROJECT_ID = RegistryEnvironment.get().config().getProjectId();
@@ -81,7 +83,7 @@ public class MetricsTaskServlet extends HttpServlet {
     ImmutableMap.Builder<String, Object> b = new ImmutableMap.Builder<>();
 
     @SuppressWarnings({"cast", "unchecked"})  // Return type is always a Set<String>.
-    Set<String> parameterKeys = (Set<String>) req.getParameterMap().keySet();
+    Set<String> parameterKeys = req.getParameterMap().keySet();
 
     for (String key : Sets.difference(parameterKeys, filter)) {
       b.put(key, req.getParameter(key));