The "end" of the day is a symbol. Any time the end of a day or the end of a season (especially winter) is invoked in poetry, there is the possibility that it symbolizes death. It isn't always the case, but here it certainly is a symbol for death. Also notice that the narrator wishes that the boy be spared a half hour at the end of the day to play. "Call it a day, I wish they might have said/ To please the boy by giving him the half hour/ That a boy counts so much when saved from work." (10-12). Had the boy been given that half hour, his life would have been spared. Since he was made to work the full day, you could say he took on the role of an adult and therefore, his childhood had also symbolically ended. This is an end of a stage of life. The end of the day and the end of this boy's childhood are both symbolic of finality and foreshadow the boy's death.
But, as loosely defined as metaphor can be, I wouldn't say that the use of "end" of the day is a metaphor for death. In this poem, the end of the day just means the end of the day. Therefore, the end of the day is symbolic of death, it's foreshadowing, but not metaphoric.
If the narrator had said, "and the boy's day ended," then 'day' would be a metaphor for 'life' and 'ended' would be a metaphor for 'death' because, although he said 'day' and 'end,' he meant 'life' and 'death.'