Map fields are represented as (synthetic) child messages of the message that contains the field. Then in the Mustache templates we need to ignore them:

https://github.com/googleapis/google-cloud-rust/blob/4f1dadcd3d7f31f155d9147774fe5f7dcb4694c6/generator/internal/rust/templates/common/message.mustache#L16

We should consider adding these messages to the MessageById table, but not to the child messages. We can simplify the mustache templates, and avoid thinking about these so much.