chore: put the pool_size default value to avoid hot upgrade failure

apps/emqx_exhook/src/emqx_exhook_mngr.erl

@@ -300,7 +300,9 @@ put_pool_size(Val) ->
     persistent_term:put({?APP, pool_size}, Val).
 
 get_pool_size() ->
-    persistent_term:get({?APP, pool_size}).
+    %% Avoid the scenario that the parameter is not set after
+    %% the hot upgrade completed.
+    persistent_term:get({?APP, pool_size}, erlang:system_info(schedulers)).
 
 save(Name, ServerState) ->
     Saved = persistent_term:get(?APP, []),

apps/emqx_exhook/test/emqx_exhook_demo_svr.erl

@@ -295,7 +295,7 @@ on_session_terminated(Req, Md) ->
      | {error, grpc_cowboy_h:error_response()}.
 on_message_publish(#{message := #{from := From} = Msg} = Req, Md) ->
     ?MODULE:in({?FUNCTION_NAME, Req}),
-    io:format(standard_error, "fun: ~p, req: ~0p~n", [?FUNCTION_NAME, Req]),
+    %io:format("fun: ~p, req: ~0p~n", [?FUNCTION_NAME, Req]),
     %% some cases for testing
     case From of
         <<"baduser">> ->