Set ndcg to default for LTR. (#8822)

- Add document.
- Add tests.
- Use `ndcg` with `topk` as default.

src/tree/updater_quantile_hist.cc

@@ -419,6 +419,7 @@ class HistBuilder {
 
   CPUExpandEntry InitRoot(DMatrix *p_fmat, linalg::MatrixView<GradientPair const> gpair,
                           RegTree *p_tree) {
+    monitor_->Start(__func__);
     CPUExpandEntry node(RegTree::kRoot, p_tree->GetDepth(0));
 
     std::size_t page_id = 0;
@@ -475,12 +476,14 @@ class HistBuilder {
       node = entries.front();
     }
 
+    monitor_->Stop(__func__);
     return node;
   }
 
   void BuildHistogram(DMatrix *p_fmat, RegTree *p_tree,
                       std::vector<CPUExpandEntry> const &valid_candidates,
                       linalg::MatrixView<GradientPair const> gpair) {
+    monitor_->Start(__func__);
     std::vector<CPUExpandEntry> nodes_to_build(valid_candidates.size());
     std::vector<CPUExpandEntry> nodes_to_sub(valid_candidates.size());
 
@@ -508,6 +511,7 @@ class HistBuilder {
                                     nodes_to_sub, gpair.Values());
       ++page_id;
     }
+    monitor_->Stop(__func__);
   }
 
   void UpdatePosition(DMatrix *p_fmat, RegTree const *p_tree,
@@ -525,6 +529,7 @@ class HistBuilder {
                      std::vector<bst_node_t> *p_out_position) {
     monitor_->Start(__func__);
     if (!task_->UpdateTreeLeaf()) {
+    monitor_->Stop(__func__);
       return;
     }
     for (auto const &part : partitioner_) {