That was a bad comment.  `PreaggregatedRollup` was a class that existed for
a while and was replaced with `PreagregatedMetric`.  In general, we
serialize simple types and rollups, but never metrics (instances of
`IMetric` now).  I had to keep reminding myself of that convention, thus
the comment.

Fixed in 4eff65a8f8faf6a289c0869523714f10e7589791.