code: def get_metadata(engine_name=None):
      
      dispatch.get(None, 'load_models')
  
      engine = engine_manager[engine_name]
      
      for tablename, m in engine.models.items():
          get_model(tablename, engine_name, signal=False)
          if hasattr(m, '__dynamic__') and getattr(m, '__dynamic__'):
              m.table.__mapping_only__ = True
      return engine.metadata
docs: get metadata according used for alembic
      It'll import all tables