Error 429 Too Many Requests when App Runner is scaling out

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## Tell us about your request
App Runner's documentation should be more transparent and warn users about the potential traffic drops due to error 429 Too Many Requests when App Runner is scaling out so that users can adjust the max concurrency value and other configurations accordingly.

## Describe alternatives you've considered
I suppose App Runner can always be better and faster at scaling out, and the max concurrency value can be higher.

## Additional context
We eventually mitigated this by:
- Setting a longer request timeout for the platform that is sending requests to the App Runner instances.
- Setting the max concurrency value to the maximum (200) in auto scaling settings.
- Increasing the minimal size in auto scaling settings (while lowering the vCPU & memory in service settings).

Please see the attachments for more information.

## Attachments
Metrics from our App Runner:

[Information shared by someone from AWS](https://www.reddit.com/r/aws/comments/174vwzq/comment/k4bufy5/?utm_source=share&utm_medium=web2x&context=3):

Review the App Runner documentation covering scaling out, auto scaling settings, max concurrency, request timeout, and HTTP 429 responses. Confirm how current guidance describes traffic drops during scale-out; done means clearly documenting the risk and the relevant configuration mitigations, then verifying the warning is understandable.

Written by the indexing model from the issue text.