A Postman test is written using tests which contain assertions. The most of the assertions are using the ChaiJS library.
At this level, we can only discuss of coverage in terms of (for example) which endpoints, methods and status codes are covered. For this, you first need to have a way of knowing this in advance and a way to track which aspects were tested.
While is it possible to use the new API tab and to provide a OpenAPI schema, it is not possible to track the coverage.