Consider first the most basic definition of metaphor: language or description that presents a comparison, not meant to be taken literally. Is Maya Angelou talking about physically rising ("like dust," or "like air")? The simple answer is no. Therefore, you can conclude that the title is definitely a metaphor.
The meaning of this metaphor lies not only in the poem itself, but in the attitude and message of most of Angelou's poetry. She writes of oppression, not just of herself, but of African Americans, and sometimes women, as a whole. This poem addresses its audience directly as "you," and it is clear the audience is supposed to be the oppressors.
First, the speaker lists many different forms of physical and emotional oppression:
You may write me down in history
With your bitter, twisted lies,
You may trod me in the very dirt
But at the end of each stanza (and repeated several times at the end of the poem) the words "I'll rise" serve as both a victory cry as well as a chant of encouragement to others. This metaphor is a reminder to emotionally rise above oppression and humiliation. It is a reminder to rise above the oppressors in maturity and take the high road. It is also a proclamation of the many who have succeeded in doing exactly this, and a battle cry to continue as long as the oppression continues.