In a jokey football context, the film "The Full Monty" used a metaphor for everyone waving their arms arms in the air : "It's the Arsenal offside trap, isn't it. Everybody holds their arms in the air and moves forwards in a line!" It sounds a little like you are describing the audience at a music event. "I was in a swamp of armpits,. as everybody stretched up for something just beyond our grasp." I was encircled and dwarfed by a forest of arms, all stretching up like so many ostriches facing a different reality." Hmm. I think the point with metaphors (and similes) is to find something fresh to say, which connects up meaning and similarity bewtween different events or ideas in a way that makes your reader better understand you. https://literary-devices.com/content/metaphor/ https://www.quora.com/Why-do-writers-use-metaphors

You're right - none of those are correct. But what you could say is 'a forest of upstretched arms'. It isn't exactly a metaphor, though. It's just a figurative use of the word 'forest', like saying 'a sea of smiling faces'.