This PR marks system_replicated_keys as a system keyspace.
It was missing when the keyspace was added.
A side effect of that is that metrics that are not supposed to be reported are.
Fixes#27903Closesscylladb/scylladb#27954
* github.com:scylladb/scylladb:
distributed_loader: system_replicated_keys as system keyspace
replicated_key_provider: make KSNAME public