Run parallel analyses

`$facet` sends the same input stream through several named sub-pipelines. It is ideal when one request must return multiple coordinated views.

For completed orders, return `byChannel` revenue summaries and the top two `byRegion` summaries in one document.

Expected result: One document with two report arrays.

[
  { $match: { status: "completed" } },
  { $facet: { byChannel: [], byRegion: [] } }
]