Mapping field meta not working

Hi,

I need to create a meta data for the field mapping as mentioned in this url: https://www.elastic.co/guide/en/elasticsearch/reference/7.x/mapping-field-meta.html

But it gives a 400 error, as below. Please let me know what is wrong in the below query?

My request

PUT my_index
{
  "mappings": {
    "properties": {
      "latency": {
        "type": "long",
        "meta": {
          "unit": "ms"
        }
      }
    }
  }
}

Response

{
  "error": {
    "root_cause": [
      {
        "type": "mapper_parsing_exception",
        "reason": "Mapping definition for [latency] has unsupported parameters:  [meta : {unit=ms}]"
      }
    ],
    "type": "mapper_parsing_exception",
    "reason": "Failed to parse mapping [_doc]: Mapping definition for [latency] has unsupported parameters:  [meta : {unit=ms}]",
    "caused_by": {
      "type": "mapper_parsing_exception",
      "reason": "Mapping definition for [latency] has unsupported parameters:  [meta : {unit=ms}]"
    }
  },
  "status": 400
}

Regards,
Sushama