Make aggregated values available for labels

Use case: We want to make a cluster map. We can use our clusterCount function for that, for example:

const viz = new carto.Viz(`
  @c_count: clusterCount()
  
  color: red
  width: @c_count
  resolution: 16
`);

It’s a common convention for cluster maps to be labeled with the number of features represented by the cluster marker:

However we can’t use the clusterCount in a label.

Right now VL requires us to use an original dataset attribute for labels instead, like in this example: https://carto.com/developers/carto-vl/examples/#example-point-labels

const viz = new carto.Viz(`
  @c_count: clusterCount()
  @v_features: viewportFeatures($poi_name)
  
  color: red
  width: @c_count
  resolution: 16
`);

....
const features = viz.variables.v_features.value;
...
"label_field": `${f.properties['poi_name']}`,

Can we create a way to use aggregated values for labels, so something like this would work?

const viz = new carto.Viz(`
        color: fuchsia
        width: 6
        strokeColor: white
        strokeWidth: 2
        filter: $major == "YES"
        @cc: clusterCount()
        @v_features: viewportFeatures($poi_name, @cc)
      `);

[elenatorro]

Merged in master ✨ We’ll have to document it properly and research how can we improve the labels visualization. Meanwhile, we’ve added a couple of examples 👍

[ramiroaznar]

Also, you can use Gencat data as I did in this example (not working because of the API Key)

https://bl.ocks.org/ramiroaznar/f856209982304290f1152828a7bf7f2b