A metaphor is a comparison. This is a story leans heavily on descriptive details instead of metaphor, using imagery—description using the five senses of sight, sound, touch, taste, and smell—to paint a picture of the scene going on, especially at the beginning of the tale when Mr. Malone arrives home from work.
The frozen lamb chop, an important item in the story, is described using a metaphor likening it to a "steel club." This murder weapon, unknown to the police, is also compared to a "sledgehammer." In the latter case, the police use a simile, a comparison employing the words "like" or "as," saying it was "like" a "sledgehammer" smashed Mr. Malone's skull. A simile is a subset of the larger category of metaphor.
Mary compares her husband, before his death, to the sun, warming her with the glow of his presence. This is an important metaphor as it provides a contrast to the ice-cold lamb chop she whacks him after he tells her he is leaving her. The frozen lamb chop functions in a larger sense, therefore, as a metaphor for how utterly chilled and hard she becomes once her husband withdraws his love. The title "lamb to the slaughter" is also a metaphor, comparing Mary, an innocent, to a sacrificial lamb her husband is about to destroy to promote his own interests.