proposal: add an additional 'final unloading' page exist observable

[janeklb]

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Motivation

Any data sent by clients from their own registered beforeunload listeners will not make it into the intake because this library's beforeunload listener is registered first and therefore executes first.

Changes

Send an additional page exit event (with a new reason: 'final_before_unload') from a setTimeout(..., 0) (seems like the best option since process.nextTick is not available in browsers, and setImmediate is not widely available in browers either).

This will result is a "best effort" of sending more data, but it's still possible to not catch everything

Testing

Not done any yet -- this is just a proposal

• [ ] Local
• [ ] Staging
• [ ] Unit
• [ ] End to end

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I have gone over the contributing documentation.

[bits-bot]

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