What is the allegory and what are the metaphors in "Stopping by Woods on a Snowy Evening?"
I'm not sure I would call Frost's "Stopping by Woods on a Snowy Evening" allegorical in any sense. One might say it's symbolic or that it involves an extended metaphor, but I don't see it as an allegory.
If the poem is symbolic or involves an extended metaphor, it is in the sense that the absent land owner, separated from nature, symbolizes humans who are separated from nature and don't realize what they're missing. Connected to this interpretation is the opposition of the man-made (such as the barn), with the natural (the snow and woods). The speaker/character, too, though he appreciates the natural, cannot stay to admire it because of human responsibilities.
The poem may also close with a metaphor:
And miles to go before I sleep,
And miles to go before I sleep.
Some commentators suggest this is a metaphor for death. In this interpretation, though the speaker longs for the peace of death (sleep), he chooses to fulfill his responsibilities and promises, rather than to seek what he wishes.
The allegory that you might be interpreting in "Stopping by Woods on a Snowy Evening" is one of obligations that a person has that should be done before the end of the day or the end of their life. What obligations or responsibilities do you feel the pressure to come back to at the end of a day—cooking, children, pets, taking care of your family? When are the “promises” we need to keep made explicit, and when do they remain unspoken? The evening is “the darkest evening of the year,” winter solstice. It is also the shortest, in a period of cold and darkness. The images of the frozen lake, the dark, the deep, could be used to argue that Frost is thinking of death. Death here is beckoning, an escape from care. The repeated lines at the end seem to reinforce the heavy sense of obligation. They make the “promises” seem more weighty, inescapable. Therefore, while the poem is laden with images of death, the poem hearkens to life and fulfilling responsibilities before it is too late. The poem ends with the repeated phrase "...miles to go...." There is always something a person can do before it is too late. So in a sense, life is reaffirming even at the end.
The allegory ,I think is temptation, and its overcoming. The line, “The woods are lovely, dark and deep”, suggests the poet’s temptation to the physical beauty of this world .Perhaps behind the woods there is a beautiful lady who the poet often meets beyond the notice of the owner .The poet’s conscience warns him .He feels the truth , and restores himself from getting involved to the alluring beauty of either the mistress or the forest .
This allegory seems to me a universal one. For, most of us in our ways of life, while enjoying a glimpse of beauty , forget our duty , and are attached with sense pleasure .
The metaphor in my opinion finds expression in the refrain , “And miles to before I sleep” .Here the implied meaning is that , art is long , and time is short . Time-wasting is a sin .