Is the glass half empty or half full? is an idiom.
The question is a common expression with both an obvious meaning and a meaning which is inferred or implied.
The phrase describes two different ways of understanding the meaning of a situation.
The idiom is used rhetorically to indicate that a particular situation could be a cause for optimism (half full) or pessimism (half empty).
Metaphor
The phrase is a metaphor. For example, According to musician Jelly Roll Morton, "If a glass of water is full, you can't add any more water, but if you have half a glass, you can always put more water in it -- and jazz music is based on the same principles."