Revert "Revert "Collect ra open file stats""

This reverts commit 1770a2178e4f778f9c99847a5159459ffbdace0e.

deps/rabbitmq_management_agent/src/rabbit_mgmt_external_stats.erl

@@ -42,7 +42,7 @@
                      rates_mode, uptime, run_queue, processors, exchange_types,
                      auth_mechanisms, applications, contexts, log_files,
                      db_dir, config_files, net_ticktime, enabled_plugins,
-                     mem_calculation_strategy]).
+                     mem_calculation_strategy, ra_open_file_metrics]).
 
 %%--------------------------------------------------------------------
 
@@ -230,7 +230,22 @@ i(gc_bytes_reclaimed, _State) ->
     Words * erlang:system_info(wordsize);
 i(context_switches, _State) ->
     {Sw, 0} = erlang:statistics(context_switches),
-    Sw.
+    Sw;
+i(ra_open_file_metrics, _State) ->
+    [{ra_log_wal, ra_metrics(ra_log_wal)},
+     {ra_log_file_segment_writer, ra_metrics(ra_log_file_segment_writer)}].
+
+ra_metrics(K) ->
+    try
+        case ets:lookup(ra_open_file_metrics, whereis(K)) of
+            [] -> 0;
+            [{_, C}] -> C
+        end
+    catch
+        error:badarg ->
+            %% On startup the mgmt might start before ra does
+            0
+    end.
 
 resource_alarm_set(Source) ->
     lists:member({{resource_limit, Source, node()},[]},