Fix metric serialization. (#9405)

tests/cpp/test_learner.cc

@@ -215,6 +215,34 @@ TEST(Learner, JsonModelIO) {
   }
 }
 
+TEST(Learner, ConfigIO) {
+  bst_row_t n_samples = 128;
+  bst_feature_t n_features = 12;
+  std::shared_ptr<DMatrix> p_fmat{
+      RandomDataGenerator{n_samples, n_features, 0}.GenerateDMatrix(true, false, 2)};
+
+  auto serialised_model_tmp = std::string{};
+  std::string eval_res_0;
+  std::string eval_res_1;
+  {
+    std::unique_ptr<Learner> learner{Learner::Create({p_fmat})};
+    learner->SetParams(Args{{"eval_metric", "ndcg"}, {"eval_metric", "map"}});
+    learner->Configure();
+    learner->UpdateOneIter(0, p_fmat);
+    eval_res_0 = learner->EvalOneIter(0, {p_fmat}, {"Train"});
+    common::MemoryBufferStream fo(&serialised_model_tmp);
+    learner->Save(&fo);
+  }
+
+  {
+    common::MemoryBufferStream fi(&serialised_model_tmp);
+    std::unique_ptr<Learner> learner{Learner::Create({p_fmat})};
+    learner->Load(&fi);
+    eval_res_1 = learner->EvalOneIter(0, {p_fmat}, {"Train"});
+  }
+  ASSERT_EQ(eval_res_0, eval_res_1);
+}
+
 // Crashes the test runner if there are race condiditions.
 //
 // Build with additional cmake flags to enable thread sanitizer