Extract make metric name from ranking metric. (#8768)

- Extract the metric parsing routine from ranking.
- Add a test.
- Accept null for string view.

tests/cpp/common/test_ranking_utils.cc

@@ -0,0 +1,38 @@
+/**
+ * Copyright 2023 by XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+
+#include <cstdint>  // std::uint32_t
+
+#include "../../../src/common/ranking_utils.h"
+
+namespace xgboost {
+namespace ltr {
+TEST(RankingUtils, MakeMetricName) {
+  std::uint32_t topn{32};
+  bool minus{false};
+  auto name = MakeMetricName("ndcg", "3-", &topn, &minus);
+  ASSERT_EQ(name, "ndcg@3-");
+  ASSERT_EQ(topn, 3);
+  ASSERT_TRUE(minus);
+
+  name = MakeMetricName("ndcg", "6", &topn, &minus);
+  ASSERT_EQ(topn, 6);
+  ASSERT_TRUE(minus);  // unchanged
+
+  minus = false;
+  name = MakeMetricName("ndcg", "-", &topn, &minus);
+  ASSERT_EQ(topn, 6);  // unchanged
+  ASSERT_TRUE(minus);
+
+  name = MakeMetricName("ndcg", nullptr, &topn, &minus);
+  ASSERT_EQ(topn, 6);  // unchanged
+  ASSERT_TRUE(minus);  // unchanged
+
+  name = MakeMetricName("ndcg", StringView{}, &topn, &minus);
+  ASSERT_EQ(topn, 6);  // unchanged
+  ASSERT_TRUE(minus);  // unchanged
+}
+}  // namespace ltr
+}  // namespace xgboost