Indices Exists API missing include_type_name in REST API spec

[russcam]

Issue

Relates: elastic/elasticsearch-net#3901

The REST API spec for indices.exists.json is missing include_type_name in 6.8.0:

elasticsearch/rest-api-spec/src/main/resources/rest-api-spec/api/indices.exists.json

Lines 16 to 42 in 1263279

	"local": {
	"type": "boolean",
	"description": "Return local information, do not retrieve the state from master node (default: false)"
	},
	"ignore_unavailable": {
	"type": "boolean",
	"description": "Ignore unavailable indexes (default: false)"
	},
	"allow_no_indices": {
	"type": "boolean",
	"description": "Ignore if a wildcard expression resolves to no concrete indices (default: false)"
	},
	"expand_wildcards": {
	"type": "enum",
	"options": [ "open", "closed", "none", "all" ],
	"default": "open",
	"description": "Whether wildcard expressions should get expanded to open or closed indices (default: open)"
	},
	"flat_settings": {
	"type": "boolean",
	"description": "Return settings in flat format (default: false)"
	},
	"include_defaults": {
	"type": "boolean",
	"description": "Whether to return all default setting for each of the indices.",
	"default": false
	}

Sending an index exists request without include_type_name results in a Deprecation Warning HTTP header being returned:

Request

HEAD http://127.0.0.1:9200/foo HTTP/1.1
Accept: application/json
Content-Type: application/json
Host: 127.0.0.1:9200
Connection: Keep-Alive

Response

HTTP/1.1 200 OK
Warning: 299 Elasticsearch-6.8.1-1fad4e1 "[types removal] The parameter include_type_name should be explicitly specified in get indices requests to prepare for 7.0. In 7.0 include_type_name will default to 'false', which means responses will omit the type name in mapping definitions."
content-type: application/json; charset=UTF-8
content-length: 225

Sending a request with include_type_name=true or include_type_name=false results in no Deprecation Warning.

Feature

The indices.exists.json REST API spec should include include_type_name in the list of params, so that clients that generate API methods from the spec pick this up.

[elasticmachine]

Pinging @elastic/es-search

[cbuescher]

@russcam instead of adding the parameter to the indices.exist API, we decided to rather prevent the deprecation log for the exists API (see discussion in #43910 and fix in #43963). Just backporting this to 6.8 now, if we do another minor release on that branch that might help the user. It doesn't really make sense to have this parameter in this API since the body is empty. Let me know if this you think this solved the issue or if we are missing something.

[russcam]

Removing the deprecation warning for index.exists is a much better solution. Thanks for the fix @cbuescher 👍