Auto shutdown in collector-exporter can hurt performance in some browsers

[kkruk-sumo]

• This only affects the JavaScript OpenTelemetry library
• This may affect other libraries, but I would like to get opinions here first

Hi 👋

@opentelemtry/exporter-collector by default adds event listener for the unload event on browser platforms.
According to MDN it can hurt website's performance when doing back/forward buttons:

Additionally, the unload event is incompatible with the back/forward cache (bfcache) implemented in modern browsers. Some browsers, such as Firefox, handle this incompatibility by excluding pages from the bfcache if they contain unload handlers, thus hurting performance.
See Avoid unload and beforeunload for more.

Also I'm not sure what's the purpose of this behaviour? shutdown only returns a promise of already exported items, so doing it on unload doesn't make more sense, does it? See the code I'm referring to.

Additionally, onShutdown is empty for node platform, so we could just remove the onInit and onShutdown functionalities. Am I missing something?

[kkruk-sumo]

CC @davidwitten and @dyladan

[MSNev]

Agree we shouldn't "shutdown" on unload, but we should perform a synchronous flush to ensure all telemetry is sent -- unless we have some sort of backing store (disk for node, and LocalStorage / IndexedDb (Slow!!!)) to ensure we can replay any unsent events on reload etc

[vanand-conga]

Is there any way I can flush synchronously?

[github-actions]

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[legendecas]

Is this still valid after #2243 is landed?

[dyladan]

I think we are still using the unload event right? So the performance issue would still be there.