Is 'it's raining cats and dogs' a metaphor?

yes and it is also an idiom it is a metaphor because there is no "like" or "as"
raining cats and dogs is not a metaphor. it is a simile.

actually, I'm pretty sure it's a idiom, a phrase that can not be taken seriously.
Yes because it is implying big objects (huge rain drops) are falling
NO ,it is an idiom to say that raining really hard