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; },