Move stale metadata check inside shard_for

metadata_cache.contains() is a slow call with many partitions becase
it scans the entire parition list looking for the specified one.

On the fetch planning path we can avoid this cost simply by
moving the check inside a shard_for check we do immediately
after the metadata check: in a stable system the shard_for check will
fail any time the metadata call will fail.

Issue redpanda-data#4410

src/v/kafka/server/handlers/fetch.cc

@@ -439,26 +439,36 @@ class simple_fetch_planner final : public fetch_planner::impl {
               auto ntp = model::ntp(
                 model::kafka_namespace, fp.topic, fp.partition);
 
-              // there is given partition in topic metadata, return
-              // unknown_topic_or_partition error
-              if (unlikely(!octx.rctx.metadata_cache().contains(ntp))) {
-                  resp_it->set(make_partition_response_error(
-                    fp.partition, error_code::unknown_topic_or_partition));
-                  ++resp_it;
-                  return;
-              }
-
               auto shard = octx.rctx.shards().shard_for(ntp);
-              if (!shard) {
-                  /**
-                   * no shard is found on current node, but topic exists in
-                   * cluster metadata, this mean that the partition was moved
-                   * but consumer has not updated its metadata yet. we return
-                   * not_leader_for_partition error to force metadata update.
+              if (unlikely(!shard)) {
+                  /*
+                   * If the shard associated with the ntp isn't found on the
+                   * current node then either (1) the ntp doesn't exist at all
+                   * (e.g., it has been deleted) or (2) it has been moved and
+                   * the client's metadata is stale.
                    */
-                  resp_it->set(make_partition_response_error(
-                    fp.partition, error_code::not_leader_for_partition));
+
+                  error_code ec;
+                  if (!octx.rctx.metadata_cache().contains(ntp)) {
+                      /*
+                       * Case (1) the given topic/partition does not exist in
+                       * topic metadata, return unknown_topic_or_partition error
+                       */
+                      ec = error_code::unknown_topic_or_partition;
+                  } else {
+                      /*
+                       * Case (2): no shard is found on current node, but topic
+                       * exists in cluster metadata, this mean that the
+                       * partition was moved but consumer has not updated its
+                       * metadata yet. we return not_leader_for_partition error
+                       * to force metadata update.
+                       */
+                      ec = error_code::not_leader_for_partition;
+                  }
+
+                  resp_it.set(make_partition_response_error(fp.partition, ec));
                   ++resp_it;
+
                   return;
               }