[FIX] do throttle when mds operation is finished instead of allocating mds memory

unittest/storage/tx/it/test_register_mds.cpp

@@ -32,6 +32,9 @@ using namespace share;
 static ObSharedMemAllocMgr MTL_MEM_ALLOC_MGR;
 
 namespace share {
+
+ObMdsThrottleGuard::~ObMdsThrottleGuard() {}
+
 int ObTenantTxDataAllocator::init(const char *label)
 {
   int ret = OB_SUCCESS;