net: add rate-rejected connection metric

Add connections_rejected_rate_limit which counts connections rejected
due to the rate limit, analogously to the existing metric which counts
rejected connections due to the hitting the open connection limit.

Fixes CORE-6827.

src/v/net/probes.cc

@@ -63,6 +63,13 @@ void server_probe::setup_metrics(
             "{}: Number of connection attempts rejected for hitting open "
             "connection count limits",
             proto))),
+        sm::make_counter(
+          "connections_rejected_rate_limit",
+          [this] { return _connections_rejected_rate_limit; },
+          sm::description(ssx::sformat(
+            "{}: Number of connection attempts rejected for hitting "
+            "connection rate limits",
+            proto))),
         sm::make_counter(
           "requests_completed",
           [this] { return _requests_completed; },