Keats is indeed talking about suicide. However, in the first stanza, he cautions his reader not to embrace the "drowsy" sleep of death but instead to stand firmly in the cold light of "wakeful anguish"? Why? Because "in the very temple of Delight / Veil'd Melancholy has her sovran shrine." There is a species of "Delight" to be found in depression, but only the poet--"him whose strenuous tongue / Can burst Joy's grape against his palate fine"--is able to ferret it out.
So, Keats tells his reader, who must be a poet, to stay awake (alive) when he is depressed in order to enjoy one of the highest pleasures that exist, but that only few--the melancholy poets--can truly experience: the sight of "a morning rose," of "the rainbow of the salt sand-wave," and of "the wealth of globed peonies." For most people, these sights are nice but not very special, yet the melancholy poet--or rather simply the poet--is deeply moved by them; and this is because he is melancholy--he feels things and sees things differently than other people, and has a happiness that healthy people cannot understand.
It's good stuff.