To my way of thinking, the whole poem is metaphorical.
If you take the poem literally, there is very little metaphor in the poem. The only place where you can really argue that metaphor is being used in this line:
Yet knowing how way leads on to way
I doubted if I should ever come back.
This is metaphorical because he's not really saying that he's going to keep following various roads and be unable to ever return to the spot where he stands.
But really, the whole poem is a metaphor. The idea of forks in roads is just a metaphor for decisions that have to be made in life. They are like forks in the road in that we have to choose which direction to go.
The extended metaphor of this poem is that of a road in the forest which forks into two paths; the path is the journey of life, and the forks are symbolic of the choices people make (or don't make) as they live out their invidividual lives. The most excruciating part of making a choice, for many people, lies in what they may miss when forsaking the other choice--in other words, what they miss on "the road not taken." Frost describes that feeling of trying to predict how the choice one leans away from might turn out: "long I stood/ And looked down one as far as I could/To where it bent in the undergrowth." He also speaks metaphorically of how one will likely not get a chance to undo what choices he or she makes: "Yet knowing how way leads onto way/I doubted that I should ever come back."