chore: App metrics no hub no promise manager

[tiina303]

Problem

I want to use app metrics for webhooks, which don't have hub.

1. We don't need a hub.
2. We also don't want to use promiseManager (it makes things unpredictable and hard to debug).
3. we weren't flushing the app metrics on app shutdown
4. removed the no longer relevant isAvailable stuff (it's 100% enabled on cloud and also that was more for the self-hosted model)
5. added logs to keep track of how long flushes are taking to make sure they aren't causing problems

Changes

6. Added an ability to flush by key limit too - giving us more options to configure when flushes happen. Too big of a queue is a problem too, e.g. when we're getting lot's of errors as each of these is a separate row currently.

The guard for not flushing in tests is because of our tests not being properly set-up isolated, ideally we'd move the app metrics completely outside of the hub, but that's for the future.

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How did you test this code?

[tomasfarias]

Nice, no hub 🥳

[tomasfarias]

lgtm, happy to see less hub

[tomasfarias]

nit: This adds an extra attribute that we have to keep track on with every method that manipulates the queue. Should be fine for now, but it could be bug-prone as it introduces a problem we didn't have: Forgetting to update the queueSize.

If this was Python I would suggest using a property that does the equivalent Object.keys(queuedData).length call, e.g.:

@property
def queue_size(self):
    return len(self.queuedData)

But with js I'm not sure if there's anything better.

The issue grows only if we add new methods that manipulate the queue, so when we do that we can tackle this too.