american english - Can a phrase be both a metaphor and an idiom?

A friend and I were discussing if the idiom 'rain check', as in 'taking a rain check' could be considered a metaphor.

We both agree that this phrase satisfies the common idiom and metaphor criteria when used in a position where it is not literally applicable, that it is still able to take on and express a legitimate, logical idea (i.e. have valid meaning.)

We agree that the difference between an idiom and a metaphor is that a metaphor requires consideration of its surrounding textual context in order to have meaning; while an idiom is a metaphor so commonly used that it has valid meaning to those unaware of its original context.

For example, a 'rain check' started off as a metaphor in local areas where the literal rain check for baseball games gave context to its use in situations outside of baseball games (e.g. I'd have to take a rain check on that BBQ). Outside of these areas, where literal rain checks in baseball set the context, the phrase 'take a rain check' would have no valid meaning.

However, once the metaphor become more popular and commonplace, many English-speaking culture began to use and understand the phrase, without the benefit of its original literal baseball context. At this point, we agree that the phrase has become an idiom.

Our point of disagreement is: does a phrase stop being a metaphor once it has become an idiom? Or do idioms retain their metaphor 'status' on promotion to an idiom?

(I tagged this post under both American and British English as I believe the phrase originated under the former, but spread to later become and idiom in the latter; British English is also what my friend and I speak and thus where we encountered this problem)