[Fusion] Allow header propagation for the Accept header

Product

Hot Chocolate

Is your feature request related to a problem?

We have a Fusion gateway in front of several Hotchocolate subgraphs.

We are using v13.9.6, which requires that the data field not be null. Because of this, if the request failed for fatal reasons: Validation, Auth, etc, the Data object would be null and if using the latest GraphQL Spec this would result in a 500 http status response.

To work around this, we decided to use the legacy spec by adding the header accept: application/json and then adding that header to our HeaderPropagation configuration.

However, we discovered that Fusion is hardcoding the Accept header to be application/graphql+json when making calls from the gateway to the subgraphs.

Our hacky workaround:
We register a DelegatingHandler on each HttpClient for the subgraphs, and we force the header to be whatever the client passed up even though Fusion is trying to hard-code it.

The solution you'd like

If Accept is listed in the HeaderPropagation options, then propagate it through to the subgraphs.

Start by tracing Fusion gateway requests to subgraphs and the HeaderPropagation options, focusing on the DelegatingHandler and HttpClient behavior described in the issue. Check where the gateway sets the Accept header and verify that configuring Accept for propagation allows the client’s value to reach subgraphs without the workaround.