## Summary
Resolves#212919
We noticed that setting the header `'Content-Type':
'text/event-stream',` didn't work as the browser's native EventSource
implementation.
```JS
return res.ok({
headers: {
'Content-Type': 'text/event-stream',
'Cache-Control': 'no-cache',
},
body: observableIntoEventSourceStream(events$ as unknown as Observable<ServerSentEvent>, {
signal: abortController.signal,
logger,
}),
});
```
The reason, apparently, is that we need to flush the compressor's buffer
negotiated in the HTTP request.
### How to test it:
Run Kibana with examples `yarn start --no-base-path --run-examples
--http2` and open the SSE example app in Kibana. You should see a clock
updating every second in the UI (the clock is coming from the server).
---------
Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
