How do idioms and metaphors fit into the principle of compositionality?

As @jlawler points out, by their very definition, idioms are non-compositional. And we could think of metaphors as a sort of non-conventional idioms. Of course, there are many other types of non-literal language such as irony or in fact any speech act, that will present similar problems.

To preserve the principle of compositionality, you can shove these into the realm of pragmatics (which is where speech acts are typically kept anyway). So expressions like 'kick the bucket' or 'take X for a ride' are simply treated as having a literal meaning where the compositionality still applies.

You could also expand your definition of what constitutes a 'component' or a syntactic rule. Neither of these is very practicable when dealing with actual range of language use but they can keep the principle intact enough for the purpose of formal semantics.

But the key thing to understand is that 'the principle of compositionality' is not some sort of a universal law of language. It is really only a way to formulate certain axioms for the purposes of formal syntax and semantics. It has no counterpart in natural semantics or in cognitive semantics because there is no need for it.