[Help] How to access document metadata?

I was able to setup import from postgress via `abc import` to ElasticSearch 7.7.
I would like to configure parent / child relationship so that I can use join queries. For this to work, http requests to Elastic must contain `routing` argument (requests with parent/child relationship without routing fails). This argument should contain a string that is always the same for all documents in a relation so that these documents can be stored in the same shard.

`abc import` does imports by sending `_bulk` requests to elastic. The first part of the _bulk item contains document metadata (_id, _type, ...) and the second part contains the actual document. Elastic alternatively accepts `routing` property in document metadata. What I haven't found is how can I access document metadata in transform function.

The transform function allows you to modify a document. Can I somehow access the document metadata and change it during import?