KTOR-7908 Add heartbeat to SSE

[marychatte]

Subsystem
Server, SSE

Motivation
KTOR-7908

[bjhham]

Couple a minor things, looks nice 👍

[bjhham]

Maybe instead of using delays and asserts here, you could use some async processing and count a few of each to cover receiving multiple...

Something like:

    @Test
    fun testHeartbeat() = testApplication {
        install(SSE)
        routing {
            sse {
                heartbeat {
                    duration = 10.milliseconds
                    event = ServerSentEvent("heartbeat")
                }

                while(true) {
                    send("Hello")
                    delay(10.milliseconds)
                }
            }
        }

        val client = createSseClient()
        
        var hellos = 0
        var heartbeats = 0
        withTimeout(1_000) {
            client.sse {
                incoming.collect { event ->
                    when(event.data) {
                        "Hello" -> hellos++
                        "heartbeat" -> heartbeats++
                    }
                    if (hellos > 3 && heartbeats > 3)
                        cancel()
                }
            }
        }
    }

[marychatte]

Looks better, thanks!

[bjhham]

Could you use Job(call.coroutineContext[Job]) here to attach it to the call?

[osipxd]

Just wondering why we don't add a CoroutineName to launched coroutines? In theory, this would simplify further debugging. Both, for us and for users.
@marychatte, @bjhham, WDYT?

[osipxd]

I know it's a bit late, but I left a coupe of comments 😅

[osipxd]

Should we add a default value here?

Suggested change

	public fun ServerSSESession.heartbeat(heartbeatConfig: Heartbeat.() -> Unit) {
	public fun ServerSSESession.heartbeat(heartbeatConfig: Heartbeat.() -> Unit = {}) {

[osipxd]

Probably period would be better?