Connections should expose a totalCount field, in order to know how many elements there are in total.

It is already supported, and needs to be added as a connectionProperties entry:

DomainContentByIdentifierConnection:
    type: relay-connection
    config:
        connectionFields:
            totalCount:
                type: Int

However, this doesn't work for some reason (might be a bug in the package). Adding it to each connection type, using the generator, would work, but requires that support for connectionFields is added to the generator API.