SSE implementation that sheds stuck clients (#14413)

* sse implementation that sheds stuck clients

* Radek and James feedback

* Refactor event streamer code for readability

* less-flaky test signaling

* test case where queue fills; fixes

* add changelog entry

* james and preston feedback

* swap our Subscription interface with an alias

* event.Data can be nil for the payload attr event

* deepsource

---------

Co-authored-by: Kasey Kirkham <kasey@users.noreply.github.com>

async/event/subscription.go

@@ -28,25 +28,6 @@ import (
 // request backoff time.
 const waitQuotient = 10
 
-// Subscription represents a stream of events. The carrier of the events is typically a
-// channel, but isn't part of the interface.
-//
-// Subscriptions can fail while established. Failures are reported through an error
-// channel. It receives a value if there is an issue with the subscription (e.g. the
-// network connection delivering the events has been closed). Only one value will ever be
-// sent.
-//
-// The error channel is closed when the subscription ends successfully (i.e. when the
-// source of events is closed). It is also closed when Unsubscribe is called.
-//
-// The Unsubscribe method cancels the sending of events. You must call Unsubscribe in all
-// cases to ensure that resources related to the subscription are released. It can be
-// called any number of times.
-type Subscription interface {
-	Err() <-chan error // returns the error channel
-	Unsubscribe()      // cancels sending of events, closing the error channel
-}
-
 // NewSubscription runs a producer function as a subscription in a new goroutine. The
 // channel given to the producer is closed when Unsubscribe is called. If fn returns an
 // error, it is sent on the subscription's error channel.