Alexandra Metz, who's also been on "Gossip Girl," says last summer ... the bouncer grabbed her, and forced her into the elevator at Upstairs -- the hotel's popular rooftop lounge and bar. There's more ... according to the suit, the guy twisted her wrist, put her in a choke hold and threw her to the floor of the lift.

She says she was injured and humiliated to the point she suffers anxiety attacks. In the suit, Alexandra doesn't offer any explanation for what sparked the alleged ass whooping.

Metz's attorney, Neil Steiner, tells us his team is narrowing in on the bouncer's identity.