Documentation on how to use QueryRoot to expose _service

[reduxdj]

I am trying to integrate ApolloRouter in rust with a subgraph written in async-graphql, do you have any examples how to expose the _service query for apollo rover, Looks like the QueryRoot type (not a pub crate) has this functionality, just don't know how to use it please.

[reduxdj]

The documentation could have this addition to make it easier to use, please.

    let schema = Schema::build(Query, EmptyMutation, EmptySubscription)
        .enable_federation()
        .finish();