Metrics alert based on ratio

We have a Spring Boot application and push its metrics to elastic, among others also usage of API paths. This generates one document per uri (path) and outcome with count per time unit.
Example: {uri="api/v2/dosomething", outcome="success", count=8}, {uri="api/v2/dosomething", outcome="server_error", count=1}), but there is no total number of requests in either document. We need to create an alert based on the ratio of errors in the total for specific paths.
I was able to create a visualisation (table) containing this information, but I cannot figure out how to create an alert based on that.
Any help?
When creating a "metrics alert", there is the option of "Rate", but this will only allow for the selection of one field and I couldn't figure out what this option really calculates. Does anyone use this somewhere and could explain the "maths" behind this?

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