Fail configuration property metadata processing when additional metadata has unexpected content

[wilkinsona]

Some additional metadata currently has no effect as it's malformed. For example #37596 is caused by a default value being declared within hints instead of properties. We should consider making such errors cause a build failure so that they can be caught early.

[wilkinsona]

We could either do this in the annotation processor and it would benefit everyone (and potentially break some people's builds if, for some reason, they want to have custom content in the file) or we could do it in our own build as an addition to CheckAdditionalSpringConfigurationMetadata and it would only benefit us but wouldn't break anyone's build.

[philwebb]

We discussed this today and we think that we should add this functionality to the annotation processor.

[mhalbritter]

I've implemented something in https://github.com/mhalbritter/spring-boot/tree/mh/37597-fail-configuration-property-metadata-processing-when-additional-metadata-has-unexpected-content