explain queries with meta do not uses index

relates to MB-12956
queries explaining queries with meta do not uses index
cbq> CREATE INDEX my_index_meta ON default(meta(type)) ;
{
"requestID": "87379929-fb09-4e35-b481-8f85dcb80c98",
"signature": null,
"results": [
],
"status": "success",
"metrics":

{ "elapsedTime": "7.028583983s", "executionTime": "7.028084518s", "resultCount": 0, "resultSize": 0 }

}

cbq> EXPLAIN SELECT meta(default) FROM default WHERE meta(default).type = "json";
{
"requestID": "895d1b7a-ae85-461a-92d8-6e43347612d2",
"signature": "json",
"results": [
{
"#operator": "Sequence",
"~children": [

{ "#operator": "PrimaryScan", "index": "#primary", "keyspace": "default", "namespace": "default", "using": "view" }

,
{
"#operator": "Parallel",
"~child": {
"#operator": "Sequence",
"~children": [

{ "#operator": "Fetch", "keyspace": "default", "namespace": "default" }

,

{ "#operator": "Filter", "condition": "((meta(`default`).`type`) = \"json\")" }

,
{
"#operator": "InitialProject",
"result_terms": [

{ "expr": "meta(`default`)" }

]
},

{ "#operator": "FinalProject" }

]
}
}
]
}
],
"status": "success",
"metrics":

{ "elapsedTime": "60.879532ms", "executionTime": "60.399859ms", "resultCount": 1, "resultSize": 1483 }

}

cbq>