[BUG] rename does not work with struct field

[currantw]

What is the bug?

The rename command returns incorrect results when applied to a struct field.

While the renamed field is correctly added to the search results, additional fields are also added that correspond to each of the fields within the struct, and which contain null values.

Consider the following input data:

name	location
Seattle	{ country: 'United States' }
Vancouver	{ country: 'Canada' }

Query: source=input | rename location as renamed_location

Expected Results:

name	renamed_location
Seattle	{ country: 'United States' }
Vancouver	{ country: 'Canada' }

Actual Results:

name	country	renamed_location
Seattle	null	{ country: 'United States' }
Vancouver	null	{ country: 'Canada' }

How can one reproduce the bug?

See comment below for sample mapping, input data, query, and results.

What is the expected behaviour?

See above. Struct field should be renamed, not additional columns should be created.

What is your host/environment?

Not applicable.

Do you have any screenshots?

Not applicable. See example above and sample mapping, input data, query, and results in comment below.

Do you have any additional context?

Discovered during work on #3030.

[currantw]

EXAMPLE

Mapping:

{
  "mappings": {
    "properties": {
      "name": {
        "type": "keyword"
      },
      "location": {
        "type": "object",
        "properties": {
          "state": {
            "type": "keyword"
          },
          "province": {
            "type": "keyword"
          },
          "country": {
            "type": "keyword"
          },
          "coordinates": {
            "type": "object",
            "properties": {
              "latitude": {
                "type": "double"
              },
              "longitude": {
                "type": "double"
              }
            }
          }
        }
      }
    }
  }
}

Data (Bulk):

{"index":{"_id":"1"}}
{"name": "Seattle", "location": { "state": "Washington", "country": "United States", "coordinates": {"latitude": 47.6061, "longitude": -122.3328}}}
{"index":{"_id":"2"}}
{"name": "Vancouver", "location": { "province": "British Columbia", "country": "Canada", "coordinates": {"latitude": 49.2827, "longitude": -123.1207}}}

Query:

source=cities | rename location AS renamed_location

Response:

{
    "schema": [
        {
            "name": "name",
            "type": "string"
        },
        {
            "name": "location.coordinates",
            "type": "struct"
        },
        {
            "name": "location.country",
            "type": "string"
        },
        {
            "name": "location.province",
            "type": "string"
        },
        {
            "name": "location.state",
            "type": "string"
        },
        {
            "name": "renamed_location",
            "type": "struct"
        }
    ],
    "datarows": [
        [
            "Seattle",
            null,
            null,
            null,
            null,
            {
                "state": "Washington",
                "country": "United States",
                "coordinates": {
                    "latitude": 47.6061,
                    "longitude": -122.3328
                }
            }
        ],
        [
            "Vancouver",
            null,
            null,
            null,
            null,
            {
                "province": "British Columbia",
                "country": "Canada",
                "coordinates": {
                    "latitude": 49.2827,
                    "longitude": -123.1207
                }
            }
        ]
    ],
    "total": 2,
    "size": 2