How to set footer via JSON config

We are using this technique for loading our grid:
It is possible to set several attributes for the header like this.

Now we are searching for setting certain footer attributes, like the demo:

However, we cannot find the “translation” of the attributes as mentioned in the demo into the JSOn style.
It must be a documentation issue. We see that header is called head in json config. What is the correct footer.

The examples to make custom functions seems too heavy.

We just want certain columns to be configured to show the sum or the total.

What are the correct JSON values.

It is not available to use the methods in json configuration,