Navigator.sendBeacon() or fetch keepalive support of unary RPC?

Hi!

I'm now trying to create a system collecting logs recorded on a page, and now wondering if there's a way to make a rpc call just before a user leaves from the page (like the last visibilitychange event or unload event).
The connection could suddenly be disconnected before sending a request because the method is called just before closing the tab and all in-flight network requests seem to be discarded when closing a tab.
Is it possible to send a request in that case?

IIUC, grpc-web seems to use [xhrio.send()](https://google.github.io/closure-library/api/goog.net.XhrIo.html) [here](https://github.com/grpc/grpc-web/blob/3956560ad01b4af0a2a5c29c081f5bbd1424e85d/javascript/net/grpc/web/grpcwebclientbase.js#L216) so it's not possible. I guess my use case can be covered by adding an option to use `navigator.sendBeacon()` or `fetch()` with a keepalive option, but not sure if the closure-library supports them.