linehaul parser for `download` should accept a `size` parameter

[miketheman]

In order to annotate how many bytes were transferred in a given download request, the upstream log needs to emit bytes that would be parsed here.

It appears that with parse_all=True, the parser format would need to be updated to allow for the existence of the new field. Does that mean duplicating MESSAGE_V3 to a MESSAGE_V4 with the added param at the end?

linehaul-cloud-function/linehaul/events/parser.py

Lines 121 to 124 in 8b8ed9d

	V3_HEADER = L("download")
	MESSAGE_v3 = (
	V3_HEADER + PIPE + REQUEST + PIPE + TLS + PIPE + PROJECT + PIPE + USER_AGENT
	)

Will need to add resp.body_bytes_written to https://github.com/pypi/infra/blob/d6702898cbed406cc431a12b80270831ef292557/terraform/file-hosting/vcl/files.vcl#L589 to emit the value when this library can parse for it.

Will likely need coordination with BigQuery team to add column when the data is correctly parsed.