fix(monitor current api): expose `disconnected_durable_sessions` count

Fixes https://emqx.atlassian.net/browse/EMQX-12423

apps/emqx_dashboard/include/emqx_dashboard.hrl

@@ -72,6 +72,7 @@
 ]).
 
 -define(GAUGE_SAMPLER_LIST, [
+    disconnected_durable_sessions,
     durable_subscriptions,
     subscriptions,
     topics,

apps/emqx_dashboard/src/emqx_dashboard_monitor.erl

@@ -262,6 +262,8 @@ merge_cluster_rate(Node, Cluster) ->
     Fun =
         fun
             %% cluster-synced values
+            (disconnected_durable_sessions, V, NCluster) ->
+                NCluster#{disconnected_durable_sessions => V};
             (durable_subscriptions, V, NCluster) ->
                 NCluster#{durable_subscriptions => V};
             (topics, V, NCluster) ->
@@ -417,22 +419,40 @@ getstats(Key) ->
         _:_ -> 0
     end.
 
-stats(connections) -> emqx_stats:getstat('connections.count');
-stats(durable_subscriptions) -> emqx_stats:getstat('durable_subscriptions.count');
-stats(live_connections) -> emqx_stats:getstat('live_connections.count');
-stats(cluster_sessions) -> emqx_stats:getstat('cluster_sessions.count');
-stats(topics) -> emqx_stats:getstat('topics.count');
-stats(subscriptions) -> emqx_stats:getstat('subscriptions.count');
-stats(shared_subscriptions) -> emqx_stats:getstat('subscriptions.shared.count');
-stats(retained_msg_count) -> emqx_stats:getstat('retained.count');
-stats(received) -> emqx_metrics:val('messages.received');
-stats(received_bytes) -> emqx_metrics:val('bytes.received');
-stats(sent) -> emqx_metrics:val('messages.sent');
-stats(sent_bytes) -> emqx_metrics:val('bytes.sent');
-stats(validation_succeeded) -> emqx_metrics:val('messages.validation_succeeded');
-stats(validation_failed) -> emqx_metrics:val('messages.validation_failed');
-stats(dropped) -> emqx_metrics:val('messages.dropped');
-stats(persisted) -> emqx_metrics:val('messages.persisted').
+stats(connections) ->
+    emqx_stats:getstat('connections.count');
+stats(disconnected_durable_sessions) ->
+    emqx_persistent_session_bookkeeper:get_disconnected_session_count();
+stats(durable_subscriptions) ->
+    emqx_stats:getstat('durable_subscriptions.count');
+stats(live_connections) ->
+    emqx_stats:getstat('live_connections.count');
+stats(cluster_sessions) ->
+    emqx_stats:getstat('cluster_sessions.count');
+stats(topics) ->
+    emqx_stats:getstat('topics.count');
+stats(subscriptions) ->
+    emqx_stats:getstat('subscriptions.count');
+stats(shared_subscriptions) ->
+    emqx_stats:getstat('subscriptions.shared.count');
+stats(retained_msg_count) ->
+    emqx_stats:getstat('retained.count');
+stats(received) ->
+    emqx_metrics:val('messages.received');
+stats(received_bytes) ->
+    emqx_metrics:val('bytes.received');
+stats(sent) ->
+    emqx_metrics:val('messages.sent');
+stats(sent_bytes) ->
+    emqx_metrics:val('bytes.sent');
+stats(validation_succeeded) ->
+    emqx_metrics:val('messages.validation_succeeded');
+stats(validation_failed) ->
+    emqx_metrics:val('messages.validation_failed');
+stats(dropped) ->
+    emqx_metrics:val('messages.dropped');
+stats(persisted) ->
+    emqx_metrics:val('messages.persisted').
 
 %% -------------------------------------------------------------------------------------------------
 %% Retained && License Quota

apps/emqx_dashboard/src/emqx_dashboard_monitor_api.erl

@@ -194,6 +194,8 @@ swagger_desc(validation_failed) ->
     swagger_desc_format("Schema validations failed ");
 swagger_desc(persisted) ->
     swagger_desc_format("Messages saved to the durable storage ");
+swagger_desc(disconnected_durable_sessions) ->
+    <<"Disconnected durable sessions at the time of sampling.", ?APPROXIMATE_DESC>>;
 swagger_desc(durable_subscriptions) ->
     <<"Subscriptions from durable sessions at the time of sampling.", ?APPROXIMATE_DESC>>;
 swagger_desc(subscriptions) ->

apps/emqx_dashboard/test/emqx_dashboard_monitor_SUITE.erl

@@ -341,6 +341,8 @@ t_persistent_session_stats(_Config) ->
     ?retry(1_000, 10, begin
         ?assertMatch(
             {ok, #{
+                <<"connections">> := 2,
+                <<"disconnected_durable_sessions">> := 0,
                 %% N.B.: we currently don't perform any deduplication between persistent
                 %% and non-persistent routes, so we count `commont/topic' twice and get 8
                 %% instead of 6 here.
@@ -356,6 +358,29 @@ t_persistent_session_stats(_Config) ->
     ?assert(PSRouteCount > 0, #{ps_route_count => PSRouteCount}),
     PSSubCount = emqx_persistent_session_bookkeeper:get_subscription_count(),
     ?assert(PSSubCount > 0, #{ps_sub_count => PSSubCount}),
+
+    %% Now with disconnected but alive persistent sessions
+    {ok, {ok, _}} =
+        ?wait_async_action(
+            emqtt:disconnect(PSClient),
+            #{?snk_kind := dashboard_monitor_flushed}
+        ),
+    ?retry(1_000, 10, begin
+        ?assertMatch(
+            {ok, #{
+                <<"connections">> := 1,
+                <<"disconnected_durable_sessions">> := 1,
+                %% N.B.: we currently don't perform any deduplication between persistent
+                %% and non-persistent routes, so we count `commont/topic' twice and get 8
+                %% instead of 6 here.
+                <<"topics">> := 8,
+                <<"durable_subscriptions">> := 4,
+                <<"subscriptions">> := 4
+            }},
+            request(["monitor_current"])
+        )
+    end),
+
     ok.
 
 request(Path) ->