test: perf: alternator: mark format string as constexpr

[tchaikov]

before this change, we use update_item_suffix as a format string fed to format(...), which is resolved to seastar::format(). but with a patch which migrates the seastar::format() to the backend with compile-time format check, the caller sites using format() would fail to build, because update_item_suffix is not a constexpr:

/home/kefu/.local/bin/clang++ -DFMT_SHARED -DSCYLLA_BUILD_MODE=release -DSEASTAR_API_LEVEL=7 -DSEASTAR_LOGGER_COMPILE_TIME_FMT -DSEASTAR_LOGGER_TYPE_STDOUT -DSEASTAR_SCHEDULING_GROUPS_COUNT=16 -DSEASTAR_SSTRING -DXXH_PRIVATE_API -DCMAKE_INTDIR=\"RelWithDebInfo\" -I/home/kefu/dev/scylladb -I/home/kefu/dev/scylladb/build/gen -I/home/kefu/dev/scylladb/seastar/include -I/home/kefu/dev/scylladb/build/seastar/gen/include -I/home/kefu/dev/scylladb/build/seastar/gen/src -isystem /home/kefu/dev/scylladb/abseil -isystem /home/kefu/dev/scylladb/build/rust -ffunction-sections -fdata-sections -O3 -g -gz -std=gnu++20 -fvisibility=hidden -Wall -Werror -Wextra -Wno-error=deprecated-declarations -Wimplicit-fallthrough -Wno-c++11-narrowing -Wno-deprecated-copy -Wno-mismatched-tags -Wno-missing-field-initializers -Wno-overloaded-virtual -Wno-unsupported-friend -Wno-enum-constexpr-conversion -Wno-unused-parameter -ffile-prefix-map=/home/kefu/dev/scylladb=. -march=westmere -mllvm -inline-threshold=2500 -fno-slp-vectorize -U_FORTIFY_SOURCE -Werror=unused-result -MD -MT test/perf/CMakeFiles/test-perf.dir/RelWithDebInfo/perf_alternator.cc.o -MF test/perf/CMakeFiles/test-perf.dir/RelWithDebInfo/perf_alternator.cc.o.d -o test/perf/CMakeFiles/test-perf.dir/RelWithDebInfo/perf_alternator.cc.o -c /home/kefu/dev/scylladb/test/perf/perf_alternator.cc
/home/kefu/dev/scylladb/test/perf/perf_alternator.cc:249:69: error: call to consteval function 'fmt::basic_format_string<char, const char (&)[1]>::basic_format_string<const char *, 0>' is not a constant expression
  249 |     return make_request(cli, "UpdateItem", prefix + seastar::format(update_item_suffix, ""));
      |                                                                     ^
/usr/include/fmt/core.h:2776:67: note: read of non-constexpr variable 'update_item_suffix' is not allowed in a constant expression
 2776 |   FMT_CONSTEVAL FMT_INLINE basic_format_string(const S& s) : str_(s) {
      |                                                                   ^
/home/kefu/dev/scylladb/test/perf/perf_alternator.cc:249:69: note: in call to 'basic_format_string<const char *, 0>(update_item_suffix)'
  249 |     return make_request(cli, "UpdateItem", prefix + seastar::format(update_item_suffix, ""));
      |                                                                     ^~~~~~~~~~~~~~~~~~
/home/kefu/dev/scylladb/test/perf/perf_alternator.cc:198:6: note: declared here
  198 | auto update_item_suffix = R"(
      |      ^

so, to prepare the change switching to compile-time format checking, let's mark this variable static constexpr. this is also more correct, as this variable is

• a compile time constant, and
• is not shared across different compilation units.

• cleanup in a test, no need to backport

[tchaikov]

@nuivall hi Marcin, could you help review this change?

[nuivall]

thanks!

[scylladb-promoter]

🟢 CI State: SUCCESS

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