KAFKA-1919: Always update the metadata, when a metadata response is received to ensure we back off.

clients/src/main/java/org/apache/kafka/clients/Metadata.java

@@ -147,6 +147,14 @@ public final class Metadata {
         log.debug("Updated cluster metadata version {} to {}", this.version, this.cluster);
     }
     
+    /**
+     * Record an attempt to update the metadata that failed. We need to keep track of this
+     * to avoid retrying immediately.
+     */
+    public synchronized void failedUpdate(long now) {
+        this.lastRefreshMs = now;
+    }
+    
     /**
      * @return The current metadata version
      */

clients/src/main/java/org/apache/kafka/clients/NetworkClient.java

@@ -381,10 +381,12 @@ public class NetworkClient implements KafkaClient {
         Cluster cluster = response.cluster();
         // don't update the cluster if there are no valid nodes...the topic we want may still be in the process of being
         // created which means we will get errors and no nodes until it exists
-        if (cluster.nodes().size() > 0)
+        if (cluster.nodes().size() > 0) {
             this.metadata.update(cluster, now);
-        else
+        } else {
             log.trace("Ignoring empty metadata response with correlation id {}.", header.correlationId());
+            this.metadata.failedUpdate(now);
+        }
     }
 
     /**