The upstream projects seem to have been working on adding support for Kotlin (at least Kotlin-JVM flavour): Protobuf in protobuf v3.17.0 and gRPC with grpc-kotlin v1.1.0.

Obviously in a pinch the Java libraries do the trick, but there are some nice integrations in the Kotlin-specific code, so it would be great if this support could be extended to these rules, once the baseline is updated.