Describe the bug
"seastar::metrics::double_registration: registering metrics twice for metrics: smf::rpc_server_active_connections" when run several rpc servers at the same time.

To Reproduce
for (int i = 0; i < 10; i ) {
smf::rpc_server_args opts;
opts.rpc_port = ports[i];
auto server = seastar::make_shared<smf::rpc_server>(opts);
...
}

To Fix
make metric name unique for each server, for example, "smf::rpc_server(" args.ip ":" seastar::to_sstring(args.rpc_port) ")" instead of "smf::rpc_server" only.