For Apache Iceberg and Delta Lake tables, we need to be able to get the size of the files written in bytes. In Iceberg, this is the required field file_size_in_bytes (docs). In Delta, this is the required field size as part of the Add action.
I think this could be exposed on FileWriter and then through that WrittenFile. But lower-level than that I'm not yet sure. FileWriter owns its OutputStream; would OutputStream::Tell() give the correct count?