Cannot add multi-field index mapping for the LongField

[yuri-rs]

I'd like to have the following index template mapping:

  "mappings": {
    "properties": {
      "long_field": {
        "type": "long",
        "fields": {
          "str": {
            "type": "keyword"
          }
        }
      },
...

Please note that I have multi-field for a Long field and it works fine.
I can create such mapping using 8.x Kibana

But I cannot create the same mapping programmatically using elastic4s, because there is no fields parameter in
LongField.scala

How can I create the mapping above using elastic4s now?
In the past (6.x) it was possible using BasicField.fields

[Philippus]

Fields are not mentioned here https://www.elastic.co/guide/en/elasticsearch/reference/current/number.html. I'm also having trouble finding it in the Elasticsearch source code.

[yuri-rs]

In java API it is defined as:
https://github.com/elastic/elasticsearch-specification/blob/d9bf86fe712f25f7ef5c0037eab484a22008c896/specification/_types/mapping/core.ts#L58

export class CorePropertyBase extends PropertyBase {
  copy_to?: Fields
  store?: boolean
}

export class DocValuesPropertyBase extends CorePropertyBase {
  doc_values?: boolean
}

export class BinaryProperty extends DocValuesPropertyBase {
  type: 'binary'
}

export class BooleanProperty extends DocValuesPropertyBase {
  boost?: double
  fielddata?: NumericFielddata
  index?: boolean
  null_value?: boolean
  type: 'boolean'
}

And PropertyBase

export class PropertyBase {
  /**
   * Metadata about the field.
   * @doc_id mapping-meta-field
   */
  meta?: Dictionary<string, string>
  properties?: Dictionary<PropertyName, Property>
  ignore_above?: integer
  dynamic?: DynamicMapping
  fields?: Dictionary<PropertyName, Property>
}

[yuri-rs]

So, it seems that any field can have fields inside based on the spec