To me, prose is a metaphor if it deliberately plays with the double-meaning, or uses it as an analogy to explain an example.
"I killed the process" isn't a metaphor to me.
"I killed the process, but its life wasn't too exciting anyway, it was just a spin-lock" would be a metaphor.
"The server's dropping packets": no metaphor.
"The server's dropping packets, and it's made a mess all over the client's carpeting.": metaphor, kinda pushing the utility, though.
If you go with the single-word definition, and care at all for etymology, almost everything is a metaphor. It's kind of fun to go on those adventures. Most of the words we use to describe cognition are metaphors for physical activities. "Understand", "Comprehend", "Grasp".
i don't think the "instead" part is strictly necessary, and the chair example is a good showcase for that. animals stand on four legs, and by analogy, we call certain chair parts "legs" too; it's clearly metaphorical in nature, it just also happens to be the "official" term
in general i think "metaphorical" is more usefully defined as sth like "re-using your knowledge of thing X to talk about thing Y, by analogy". and you might use a cool metaphor for rhetorical or poetic effect, but they're just a common element of language – see several mentions of Lakoff in other comments for examples.