HostExecAggStageAdapter is in charge of getting the upstream document BSON and sending it to the extension that requested it (done already). It should also set the metadata member on apiResult to the metadata of the document so that the extension can read from it and write to it.

This ticket should modify the logic in HostExecAggStageAdapter::_hostGetNext() so that when the hostStatusCode is kAdvanced, we:

1. Split the metadata from the document
2. Create a buffer with the metadata and set apiResult->metadata to that buffer

The process of allocating and sending a metadata buffer across the boundary should match how it's done with the document.