feat(dashboard): endpoint `/monitor_current` provides mor fields

- `retained_msg_count`
  Current retained_msg_count on each node and should be same on all nodes.

- `license_quota`
  Only for enterprise edition, provides the max limited connections num.

apps/emqx_dashboard/src/emqx_dashboard_monitor.erl

@@ -173,7 +173,9 @@ handle_call(current_rate, _From, State = #state{last = Last}) ->
     NowTime = erlang:system_time(millisecond),
     NowSamplers = sample(NowTime),
     Rate = cal_rate(NowSamplers, Last),
-    {reply, {ok, Rate}, State};
+    NonRateValue = non_rate_value(),
+    Samples = maps:merge(Rate, NonRateValue),
+    {reply, {ok, Samples}, State};
 handle_call(_Request, _From, State = #state{}) ->
     {reply, ok, State}.
 
@@ -409,3 +411,21 @@ stats(received_bytes) -> emqx_metrics:val('bytes.received');
 stats(sent) -> emqx_metrics:val('messages.sent');
 stats(sent_bytes) -> emqx_metrics:val('bytes.sent');
 stats(dropped) -> emqx_metrics:val('messages.dropped').
+
+%% -------------------------------------------------------------------------------------------------
+%% Retained && License Quota
+
+%% the non rate values should be same on all nodes
+non_rate_value() ->
+    #{
+        retained_msg_count => emqx_retainer:retained_count(),
+        license_quota => license_quota()
+    }.
+
+license_quota() ->
+    case emqx_license_checker:limits() of
+        {ok, #{max_connections := Quota}} ->
+            Quota;
+        {error, no_license} ->
+            0
+    end.

apps/emqx_dashboard/src/emqx_dashboard_monitor_api.erl

@@ -1,5 +1,17 @@
 %%--------------------------------------------------------------------
-%% Copyright (c) 2019-2023 EMQ Technologies Co., Ltd. All Rights Reserved.
+%% Copyright (c) 2020-2024 EMQ Technologies Co., Ltd. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
 %%--------------------------------------------------------------------
 
 -module(emqx_dashboard_monitor_api).

apps/emqx_retainer/src/emqx_retainer_api.erl

@@ -44,7 +44,11 @@ api_spec() ->
     emqx_dashboard_swagger:spec(?MODULE, #{check_schema => true}).
 
 paths() ->
-    [?PREFIX, ?PREFIX ++ "/messages", ?PREFIX ++ "/message/:topic"].
+    [
+        ?PREFIX,
+        ?PREFIX ++ "/messages",
+        ?PREFIX ++ "/message/:topic"
+    ].
 
 schema(?PREFIX) ->
     #{