In ed3e4f33fd we introduced new connection throttling feature which is controlled by uninitialized_connections_semaphore_cpu_concurrency config. But live updating of it was broken, this patch fixes it.
When the temporary value from observer() is destroyed, it disconnects from updateable_value, so observation stops right away. We need to retain the observer.
Backport: to 2025.2 where this feature was added
Fixes: https://github.com/scylladb/scylladb/issues/24557Closesscylladb/scylladb#24484
* github.com:scylladb/scylladb:
test: add test for live updates of generic server config
utils: don't allow do discard updateable_value observer
generic_server: fix connections semaphore config observer
23f0d275c8