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

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

src/common/ranking_utils.h

@@ -0,0 +1,29 @@
+/**
+ * Copyright 2023 by XGBoost contributors
+ */
+#ifndef XGBOOST_COMMON_RANKING_UTILS_H_
+#define XGBOOST_COMMON_RANKING_UTILS_H_
+
+#include <cstddef>                // std::size_t
+#include <cstdint>                // std::uint32_t
+#include <string>                 // std::string
+
+#include "xgboost/string_view.h"  // StringView
+
+namespace xgboost {
+namespace ltr {
+/**
+ * \brief Construct name for ranking metric given parameters.
+ *
+ * \param [in] name   Null terminated string for metric name
+ * \param [in] param  Null terminated string for parameter like the `3-` in `ndcg@3-`.
+ * \param [out] topn  Top n documents parsed from param. Unchanged if it's not specified.
+ * \param [out] minus Whether we should turn the score into loss. Unchanged if it's not
+ *                    specified.
+ *
+ * \return The name of the metric.
+ */
+std::string MakeMetricName(StringView name, StringView param, std::uint32_t* topn, bool* minus);
+}  // namespace ltr
+}  // namespace xgboost
+#endif  // XGBOOST_COMMON_RANKING_UTILS_H_