Put template APIs should only wrap a top level _doc json object field to the mapping if it is missing

[martijnvg]

Component and composable templates should accept mappings with and without a top level _doc json object.
If the top level _doc json field is missing then it should be added to the mapping and if it already exists then
the mapping doesn't need to be modified.

Currently in 8.0-snapshot (master) the put component and composable index templates add a toplevel _doc json
object to the mapping if it missing. However in 7.x and before this is not the case and if a mapping with _doc json
object as top level field is added then an error is returned that the root mapping as unsupported parameters. This
is because a mapping in a template is always wrapped in toplevel _doc json object field.

Composable index template case:

PUT _index_template/my-template
{
    "index_patterns": [
        "my-logs-*"
    ],
    "template": {
        "mappings": {
            "_doc": {
                "properties": {
                    "@timestamp": {
                        "type": "date"
                    }
                }
            }
        }
    }
}

The above request fails with a mapper exception (Root mapping definition has unsupported parameters: [_doc : {properties={@timestamp={type=date}}}]").

{
    "index_patterns": [
        "my-logs-*"
    ],
    "template": {
        "mappings": {
            "properties": {
                "@timestamp": {
                    "type": "date"
                }
            }
        }
    }
}

The above request succeeds.

Component template case:

PUT /_component_template/my-template
{
    "template": {
        "mappings": {
            "_doc": {
                "properties": {
                    "@timestamp": {
                        "type": "date"
                    }
                }
            }
        }
    }
}

The above request also fails with a mapper exception (Root mapping definition has unsupported parameters: [_doc : {properties={@timestamp={type=date}}}]").

PUT /_component_template/my-template
{
    "template": {
        "mappings": {
            "properties": {
                "@timestamp": {
                    "type": "date"
                }
            }
        }
    }
}

The above request succeeds.

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)

[martijnvg]

The master branch contains #58867, however this change was never back ported to the 7.x branch.

[martijnvg]

Fixed via #58969 in 7.x branch. The fix will be included in the 7.14 release.