To find quarrel in a straw is this a metaphor?

No, it is not.

Metaphors are seen in examples below:

She is a pig.

Bent is a chicken.

Jay is the horse.

It is comparing two things without using 'as' or 'like'. Direct comparison. You are comparing her to a pig; Bent to a chicken; Jay to a horse. When you use 'as' or 'like', like in the below, you are using simile not metaphor.

She is like a pig.

Bent is as a chicken.

Jay is like a horse.

Finding quarrel in a straw is an idiom. Check that out in your dictionary.

Hope I have been helpful.