we have the case where a worker can actually handle a request at any time from a client ...
we have the case where a worker can actually handle a request at any time from a client ... even before we bother subscribing to the exchange.
This isn't even a race condition ... there's nothing to say requests to a worker 'microservice' (ahem) has to have gone through our exchange at all.
Hence, the subscription key is optional
Captures the 'handler' logic for a subscription.