We saw this happening in Canada some years back (Thanks, Drew!) with the government of the day proposing ISPs being turned into attractive targets for anyone wanting to impersonate people ("identity theft").
Worse, the kind of processing required to extract the metadata requires a machine the cost of one's main router, so people proposed ISPs should "just spool everything to disk" for a few days.
The next thought was to call for a longer retention period...
[It didn't pass, somewhat miraculously]