Missing configuration metadata in Gateway MVC

Describe the bug

The configuration properties in the spring.cloud.gateway.mvc.routes-map group are missing metadata, resulting in IDEs not being able to validate values or suggest completions. It looks like something related to the fact that it's defined as a Map and includes an object which is defined in another class, not nested. The latter scenario would be typically solved with the @NestedConfigurationProperty annotation, but it doesn't support Maps. On the other hand, the spring.cloud.gateway.mvc.routes group (defined as a List) comes with the right metadata.

A possible solution would be to add the metadata explicitly in a additional-spring-configuration-metadata.json file. I'm not sure if there's some way in Spring Boot to support this scenario automatically via the Configuration Processor.

Start by locating the Gateway MVC configuration that defines the spring.cloud.gateway.mvc.routes-map group and compare its generated metadata with the spring.cloud.gateway.mvc.routes group. Check whether the Spring Boot configuration processor supports this Map case or whether additional-spring-configuration-metadata.json is needed. Done means IDE validation and completion work for the routes-map properties.