Owen describes the soldiers' feet in vivid terms as being "shod" in blood, suggesting that the coating of blood is thick and all-encompassing, like shoes. So, one could argue that this is a metaphor, in that the blood coating is being compared to shoes.
However, in other ways, this is simply a vivid literal description. To be "shod" in something simply means to have it on your feet, in the strictest sense. So, the soldiers are literally wearing blood on their feet. They are literally blood-shod, rather than metaphorically.
In any event, this is a clear use of the sort of vivid imagery which permeates this poem. Owen is attempting to convey to his audience the many difficulties soldiers must suffer for the sake of the "glory" they have been promised by the men in charge of the war effort. Far from being able to march home gloriously, they are instead bleeding, sick, and mentally and physically exhausted, in boots which do not fit and in some cases without boots at all. This is what Owen is saying when he describes the soldiers as being blood-shod. He very much wants it to be clear that they really are literally shod in blood, their own and possibly other people's.