Publish metrics or events when AppRunner disconnects a client due to its request timeout

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**Tell us about your request**
It would be great if AppRunner could either publish some kind of event whenever it disconnects a client due to its request timeout of 120 seconds (as of 2024 - see also #104), or if it could at least publish metrics on how often it did so. Ideally, these metrics should be shown in the Metrics panel of the AppRunner console.

**Describe alternatives you've considered**
As far as I can tell, the only way we are able to figure out if something got disconnected due to AppRunner itself vs e.g. a client disconnecting from their end is to look at the time the request took before it was disconnected - if it's around 120 seconds, it was probably done by AppRunner. This isn't very convenient or exact, though.

**Additional context**
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