Emitted immediately after writing a body chunk for a message.
Note that this signal is not parallel to [signal@ServerMessage:SoupServerMessage:got-chunk
]; it is emitted only when a
complete chunk (added with [method@MessageBody.append] or [method@MessageBody.append_bytes] has been written. To get more useful
continuous progress information, use [signal@ServerMessage:SoupServerMessage:wrote-body-data
].