we should not format a variable unless we want to print it. in this
case, we format `first_row` using `fmt::to_string()` to a string,
and then insert the string to another string, despite that this is
in a cold path, this is still a anti pattern -- both convoluted,
and not performant.
so let's just pass `first_row` to `format()`.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
4f8c1a4729