Support "fields" key in vector_layers array returned by the map capabilities endpoint

[ARolek]

Currently our map capabilities endpoint does not return a value for the "fields" key for vector_layers. At the time of writing this, the values to be included in the vectory_layers array is still being debated for the TileJSON spec. It also appears that the discussion is not aligning with Mapbox's current implementation. An example of Mapbox's implementation can be seen on issue #83.

Tegola currently supports "fields" (or as they're called in MVT, "tags") to be fetched from data providers as well as set default tags per layer. With the way Tegola is currently designed data providers would need a way to be queried for the supported "fields" by layer. This result set would then need to be combined with the default tags. The mvt.Provider interface might need to be expanded, or the LayerNames() method be replaced with Layers() and return more information about the layer (i.e. Name and Tags).

Another thing to consider is that Mapbox's current implementation of "fields" consists of key:value pairs, where the key is the name of the "field" and the value is a description. If we want to support the description we would need to figure out how to best implement this (in the config?).

[four43]

I'd like to bump this, looks like Tilejson v3 is out: https://github.com/mapbox/tilejson-spec/blob/22f5f91e643e8980ef2656674bef84c2869fbe76/3.0.0/README.md#33-vector_layers

I'm trying to create mbtiles from my tegola cache files and running into all sorts of issues. This would really help. Thanks!

[ARolek]

@four43 what types of issues are you running into that this functionality would fix?