api: Shorten get_host_to_id_map() handler

The handler does

- gets host IDs from local token metadata
- for each ID gets the host IP and generates IP:ID std::pair
- converts the sequence of generated pairs into std::unordered_map
- converts the unordered map into vector of jsonable key:value objects

This patch removes the 3rd step and makes the needed jsonable object in
step 2 directly, thus eliminating the interposing unordered_map
creation.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

Closes scylladb/scylladb#24354

api/token_metadata.cc

@@ -74,11 +74,14 @@ void set_token_metadata(http_context& ctx, routes& r, sharded<locator::shared_to
     });
 
     ss::get_host_id_map.set(r, [&tm, &g](const_req req) {
-        std::vector<ss::mapper> res;
-        auto map = tm.local().get()->get_host_ids() |
-            std::views::transform([&g] (locator::host_id id) { return std::make_pair(g.local().get_address_map().get(id), id); }) |
-            std::ranges::to<std::unordered_map>();
-        return map_to_key_value(std::move(map), res);
+        return tm.local().get()->get_host_ids()
+            | std::views::transform([&g] (locator::host_id id) {
+                ss::mapper m;
+                m.key = fmt::to_string(g.local().get_address_map().get(id));
+                m.value = fmt::to_string(id);
+                return m;
+            })
+            | std::ranges::to<std::vector<ss::mapper>>();
     });
 
     static auto host_or_broadcast = [&tm](const_req req) {