How to configure or describe `meta`?

[zerebubuth]

When querying PostgreSQL, we might want to calculate some things server-side (e.g: clip polygons, polygon centroids, label placement points) because returning the raw data in the table would unnecessarily inflate the network traffic for data we don't actually need. This is sometimes called meta in the query result processing.

RAWR tiles includes whole polygons, which means centroids and label placement points can be calculated client-side. It's not clear that we'd want to calculate the same things and embed them in the RAWR tile, as this would not only move work from the distributed tilequeue processes to the centralised RAWR tile generation process, but also increase the size of the tile and make the choices of algorithm for these calculations less flexible.

The question is whether to hard-code some of these rules, or configure them in such a way that they can be shared between RAWR tile generation and PostgreSQL query generation.