The primary metaphor is "Gather ye rosebuds while ye may." Rosebuds are youth and beauty. This is the theme of the poem, which is "carpe diem" or "seize the day." We only have one life, and one youth. It must be enjoyed while it lasts.
Just as a reminder: a metaphor is "a figure of speech in which an expression is used to refer to something that it does not literally denote in order to suggest a similarity. "