Ability to obtain the upstream service names appearing in APM's Service Map

[jmoon-elastic]

It is required for an external application to obtain the upstream service names of a target service in APM's Service Map.

For example(using the demo gallery), a query with productCatalogService to return checkoutService, frontend-node and recommendationService.

Please provide a query DSL or an API that would return all of the upstream service names for a target service. Thank you!