Custom Metrics are available for Google Ads, Microsoft Ads, and Amazon Ads. For Amazon Ads, however, the following metrics are not supported:
All Conversion Value
So if you were to select in a dropdown a custom metric that uses any of these two metrics in the formula, we will return the number of conversions as 0, and no alert will be triggered for this platform.
You can set up alerts for your custom metrics using the Alerts tool to start monitoring their performance. Read more about setting up alerts here.
To read more about how to set up these calculated metrics on the All Accounts Dashboard, head over to this article.