Add GetMetadata() method to TaskHandler interface to allow for propagating metadata to processors

[kieranbarty]

We want to be able to use optional, client defined fields for things like trackingID through our entire agent lifecycle.

Currently we can achieve this by adding context values based on user inputs, but we would like to avoid bloating the context for a given request. We could achieve this by allowing us to store the metadata on the TaskHandler itself, and we could then retrieve it as/when we need to when we process a message. I'm happy to contribute this change as well :)

Heres an example of how I think we could do this:

TaskManager.go

handle := &taskHandler{
	taskManager:       t,
	metadata: request.Metadata,
	messageId:         request.Message.MessageID,
	ctx:               ctx,
}

TaskHandler.go

type taskHandler struct {
	taskManager           *TaskManager
	messageId             string
	metadata              map[string]any
	ctx                   context.Context
}

// Optionally we can pass a key into this method, and return single values.
func (t taskHandler) GetMetadata() (map[string]any, error) {
	if t.metadata == nil {
		return nil, errors.New("metadata is nil")
	}

	return t.metadata, nil
}

[hyprh]

I think it's OK to add this field. Thanks for you contribution:)

And I wonder if the trackingID is from the A2A client? When handling http request headers, taskHandler seems unable to be accessed.

[kieranbarty]

@hyprh so when we handle a request in the TaskManager, we would expect to pull the tracking ID from the metadata field of a given request, set in the client. We would usually do this via HTTP headers like you mentioned, but in this case parameters are fine as it's still optional. We'll just generate a new tracking ID if one isn't sent in the parameters :)