What is an example of a metaphor in the story ''There Will Come Soft Rains''?

The house is compared to an altar, and the house itself is a metaphor for the dangers of over-relying on technology.

A metaphor is a common type of figurative language where something is referred to as something it is not.  Unlike a simile, a metaphor does not compare something to something else by saying it is like something else.  Metaphors say that something is something else.

This story describes a house that is the last house standing after a seriously devastating event, such as a nuclear holocaust.  The house is fully automated, and continues operating as if there were people inside it long after the people are dead.  Eventually, the house is destroyed in a fire it is unable to put out.

There are many similes in the story, but metaphor is also used to describe the house.

The house was an altar with ten thousand attendants, big, small, servicing, attending, in choirs. But the gods had gone away, and the ritual of the religion continued senselessly, uselessly.

This is a specific metaphor.  In this case, the house is compared to an altar.  An altar is a place of worship.  The metaphor compares that robots in the house to worshippers.  They are personified throughout, as if they were alive.  In this case, they are worshipping the house because it is their purpose.

The house itself is also a metaphor in a larger sense.  It is a metaphor for the destructive nature of technology.  Even though the family is dead, the house continues without them.  It was a more serious technology that destroyed all of the other houses and killed the people, but the idea is the same.  Too much technological advancement is dangerous.

At ten o'clock the house began to die. The wind blew. A failing tree bough crashed through the kitchen window. Cleaning solvent, bottled, shattered over the stove. The room was ablaze in an instant!

Just as the entire rest of the world was destroyed in a nuclear event, the house was destroyed in a fire.  Despite all of its technological advancement, the house could not save itself.  Bradbury is trying to warn us that we should not rely too much on technology.