For requests that do not have routes, I would need to write a filter that can send xDS queries to get a Route Added with the associated configuration (cluster def and endpoints).

The use case is for deployments with a very high cluster cardinality and failing the request is not an option.

I do not know if this should be an envoy feature but it does not seem like it can be implemented as a custom filter as there are not xDS headers in include/envoy.

Any pointers on how to achieve this would be greatly appreciated.

cc @mattklein123 @htuch