Field API fails with _id being an alias

[timroes]

Steps to reproduce:

1. Create the following index with the appropriate index mapping:

PUT /my-index-000001
{
  "mappings": {
    "properties": {
      "@timestamp":    { "type": "date" },  
      "pseudonym":   { "type": "alias", "path": "_id"  }
    }
  }
}

2. Create the following document:

POST my-index-000001/_doc/
{
  "@timestamp": "2021-07-29T13:13:00",
  "message": "GET /search HTTP/1.1 200 1070000",
  "user": {
    "id": "kimchy"
  }
}

3. Query now for all fields:

GET my-index-000001/_search
{
  "fields": [ "*" ]
}

The last query will now fail with the following exception:

{
  "error" : {
    "root_cause" : [
      {
        "type" : "unsupported_operation_exception",
        "reason" : "unsupported_operation_exception: Cannot fetch values for internal field [_id]."
      }
    ],
    "type" : "search_phase_execution_exception",
    "reason" : "all shards failed",
    "phase" : "query",
    "grouped" : true,
    "failed_shards" : [
      {
        "shard" : 0,
        "index" : "field-alias-test",
        "node" : "VBJ2JqanRZG4IwpqrPS-WA",
        "reason" : {
          "type" : "unsupported_operation_exception",
          "reason" : "unsupported_operation_exception: Cannot fetch values for internal field [_id]."
        }
      }
    ]
  },
  "status" : 500
}

Expected behavior: This request should not fail, but return all fields correctly.

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[cbuescher]

I looked into this a bit and we came up with a suggestion in the team that we might move away from generally prevent fetching internal fields via the fields option.
We would still exclude any metadata field from matching wildcard patterns like "*", but if explicitely asked for (or like in this case requested through an alias) we'd return the value. I opened #78828 as a proposal to discuss this option.

[cbuescher]

We implemented the above proposal for the following metadata fields: _id, _routing,
_ignored, _index and _version.
There are a few other internal fields (e.g. "_seq_no", "_nested_path", "_feature"...) that still throw an error when trying to retrieve them via an alias, but I think they are even more exotic and even less likely to be used in an alias, so I didn't include them in the work so far. I will open a follow up issue to discuss how we want to handle those (if e.g. ignoring them generally when trying to get value fetchers for them is better than throwing an error), but I think we can close this issue since requesting the above fields (directly or with an alias) should now work starting in 7.16.