Gateway should offer configuration point for introspection query

Gateways currently offer a configuration point for buildService -> willSendRequest, which allows setting headers on normal federated requests to their services, but they offer no configuration hook for sending the initial query GetServiceDefinition { _service { sdl } } request.

https://spectrum.chat/apollo/apollo-federation/sending-authorization-header-on-gateway-introspection-query~e8c3c16a-07b7-464a-97f5-13f577591bb9

[brandonpapworth]

This would be quite helpful, but I also see it as useful for us to possibly configure the actual method used to communicate with the service. One might want to use lambda.invoke as opposed to fetch, and all we would need is to provide a function that implements a similar API.

I have patched the Apollo Gateway in my own codebase to facilitate my desire for leveraging lambda.invoke (why add an API Gateway layer if I don’t need it? 😄 ) with some of the services, ensuring to construct the context object to be properly accepted by the apollo-server-lambda handler.

[ElliotChong]

I just ran into this issue today (my service requires a particular header), FWIW it would be nice to be able to manipulate headers on a per-service basis rather than just globally. Perhaps as a headers prop that can be included alongside name and url?