what is the allegory in the poem?

Allegory is a form of extended metaphor, in which objects, persons, and actions in a narrative, are equated with the meanings that lie outside the narrative itself. Much of this poem has Prufrock acting like the famously indecisive Hamlet. Hamlet, however, finally made dicusions, this guy doesn't.

Consider Lines 111-119: In this important metaphor, Prufrock likens himself to Prince Hamlet, the title character from Shakespeare’s most famous play. But then he decides he’s actually more of an "attendant lord" who could be confused for a fool, which we think is an allusion to Polonius, the father of the character Ophelia in the same play.