Feature Request: Add response context support for interceptors (esp. Subscriptions)

[michelepra]

Problem

Interceptors in gqlgen only receive the request context as their first parameter.
For queries and mutations, this works fine because execution is linear:

1. Receive request
2. Execute resolver
3. Return response (context == request context)

For subscriptions, however:

• gqlgen starts a goroutine that publishes values to a channel
• gqlgen reads from this channel and encodes responses
• At this point, the response has no associated context that interceptors can access

As a result, interceptors cannot access or enrich the "response context" (e.g., metadata, tracing, request-scoped values) for subscription payloads.
They only see the request context from when the subscription was created.

---

Proposed Changes

1. Extend Response struct with a Context field

Current:

type Response struct {
	Errors     gqlerror.List   `json:"errors,omitempty"`
	Data       json.RawMessage `json:"data"`
	Label      string          `json:"label,omitempty"`
	Path       ast.Path        `json:"path,omitempty"`
	HasNext    *bool           `json:"hasNext,omitempty"`
	Extensions map[string]any  `json:"extensions,omitempty"`
}

Proposed:

type Response struct {
	Context context.Context `json:"-"`

	Errors     gqlerror.List   `json:"errors,omitempty"`
	Data       json.RawMessage `json:"data"`
	Label      string          `json:"label,omitempty"`
	Path       ast.Path        `json:"path,omitempty"`
	HasNext    *bool           `json:"hasNext,omitempty"`
	Extensions map[string]any  `json:"extensions,omitempty"`
}

This allows interceptors to access response-level context.

2. Introduce a SubscriptionField interface for subscription result payloads

Instead of subscription resolvers returning (<-chan *T, error), have them return something like:

type SubscriptionField[T any] interface {
	GetContext() context.Context
	GetField() T
}

So subscription resolvers could return:

(<-chan SubscriptionField[*T], error)

This lets each published item carry its own context, which flows naturally into the modified Response.Context.

---

Some benefit

• Consistent interceptor behavior across queries, mutations, and subscriptions withouth break current interceptor code
• Enables request/response tracing, per-event metadata, and richer observability for subscriptions
• Clean separation of concerns: business payload stays in GetField(), while per-response metadata lives in context.Context