perf_tests: Right-align numeric metrics in markdown tables

Right-align metrics columns in markdown output for better number comparison.
Previously, metrics were left-aligned in markdown tables but right-aligned
in stdout, making it difficult to visually compare numbers.

The change adds colons to the right of hyphens in the header row
(e.g. `|---:|`) to right-align entire columns. While this affects both
headers and data cells, the improved readability of numeric values
outweighs the minor tradeoff of right-aligned headers.

In short:
- Uses `column::print_text` with ":>" alignment for consistent formatting
- Maintains existing stdout_printer right alignment
- Updates `markdown_printer` alignment to match stdout behavior

Signed-off-by: Kefu Chai <[email protected]>

tests/perf/perf_tests.cc

@@ -357,6 +357,21 @@ class json_printer final : public result_printer {
 
 class markdown_printer final : public result_printer {
     std::FILE* _output = nullptr;
+
+    void print_header_row(const char* head_text, const char* body_text) {
+        // start delimeter, and the top-left cell
+        fmt::print(_output, "| {:<{}}", head_text, name_column_length());
+        // the header cells
+        for (auto& c : text_columns) {
+            // middle delimeter
+            fmt::print(_output, " | ");
+            // right align the text in result cells
+            c.print_header(_output, body_text);
+        }
+        // end delimeter
+        fmt::print(_output, " |\n");
+    }
+
 public:
     explicit markdown_printer(const std::string& filename) {
         if (filename == "-") {
@@ -375,9 +390,10 @@ class markdown_printer final : public result_printer {
     }
 
     void print_configuration(const config&) override {
-        // print the header row, then the divider row of all -
-        print_text_header(_output, text_columns, name_column_length(), "| ", " | ", " |", "test");
-        print_text_header(_output, text_columns, name_column_length(), "| ", " | ", " |", "test", "-");
+        // print the header row
+        print_header_row("test", nullptr);
+        // then the divider row of all -
+        print_header_row("-", "-:");
     }
 
     void print_result(const result& r) override {