Who is the ''I'' in the line ''I met a traveler from an antique land'' in Percy Bysshe Shelley's poem "Ozymandias"?

This device (which we might call “the poet/narrator”) allows a story-telling element into the “lyric” process, thereby removing the single narrator (a requirement of Aristotle’s division of “Poetry” into three types – Epic, Lyric, and Dramatic –  differentiated largely by their narrative techniques).  By utilizing a first-person pronoun, Shelley allows the narration of a third party, the actual witness to the ruined statue of Ozymandias.  A contemporary equivalent might be your telling an anecdote which you heard from a third party, rather than an anecdote involving you directly.  Another famous poem that uses this technique is Coleridge’s Rhyme of the Ancient Mariner.  The technique allows the poet to condense and edit the actual recital of the anecdote, changing the order and the intensity of the original story as told to him as he had heard it, in other words to give it a creative form.