Restore FD info in rabbitmqctl status

The FD limits are still valuable.

The FD used will still show some information during CQv1
upgrade to v2 so it is kept for now. But in the future
it will have to be reworked to query the system, or be
removed.

deps/rabbit/src/rabbit.erl

@@ -751,6 +751,9 @@ status() ->
                                         get_disk_free_limit, []}},
             {disk_free,                {rabbit_disk_monitor,
                                         get_disk_free, []}}]),
+    S3 = rabbit_misc:with_exit_handler(
+           fun () -> [] end,
+           fun () -> [{file_descriptors, file_handle_cache:info()}] end),
     S4 = [{processes,        [{limit, erlang:system_info(process_limit)},
                               {used, erlang:system_info(process_count)}]},
           {run_queue,        erlang:statistics(run_queue)},
@@ -785,7 +788,7 @@ status() ->
                (_) -> false
            end,
            maps:to_list(product_info())),
-    S1 ++ S2 ++ S4 ++ S5 ++ S6 ++ S7 ++ S8.
+    S1 ++ S2 ++ S3 ++ S4 ++ S5 ++ S6 ++ S7 ++ S8.
 
 alarms() ->
     Alarms = rabbit_misc:with_exit_handler(rabbit_misc:const([]),

deps/rabbitmq_cli/lib/rabbitmq/cli/ctl/commands/status_command.ex

@@ -164,6 +164,11 @@ defmodule RabbitMQ.CLI.Ctl.Commands.StatusCommand do
           "#{category}: #{IU.convert(val[:bytes], unit)} #{unit} (#{val[:percentage]} %)"
         end)
 
+    file_descriptors = [
+      "\n#{bright("File Descriptors")}\n",
+      "Total: #{m[:file_descriptors][:total_used]}, limit: #{m[:file_descriptors][:total_limit]}"
+    ]
+
     disk_space_section = [
       "\n#{bright("Free Disk Space")}\n",
       "Low free disk space watermark: #{IU.convert(m[:disk_free_limit], unit)} #{unit}",
@@ -194,7 +199,7 @@ defmodule RabbitMQ.CLI.Ctl.Commands.StatusCommand do
         log_section ++
         alarms_section ++
         memory_section ++
-        disk_space_section ++ totals_section ++ listeners_section
+        file_descriptors ++ disk_space_section ++ totals_section ++ listeners_section
 
     {:ok, Enum.join(lines, line_separator())}
   end
@@ -258,6 +263,7 @@ defmodule RabbitMQ.CLI.Ctl.Commands.StatusCommand do
       vm_memory_high_watermark_limit: Keyword.get(result, :vm_memory_limit),
       disk_free_limit: Keyword.get(result, :disk_free_limit),
       disk_free: Keyword.get(result, :disk_free),
+      file_descriptors: Enum.into(Keyword.get(result, :file_descriptors), %{}),
       alarms: Keyword.get(result, :alarms),
       listeners: listener_maps(Keyword.get(result, :listeners, [])),
       memory: Keyword.get(result, :memory) |> Enum.into(%{}),