Index pattern always treats field named "_timestamp" as a date regardless of mapping

[danielharada]

Kibana version:
7.16.1

Elasticsearch version:
7.16.1

Server OS version:
ESS

Browser version:

Browser OS version:

Original install method (e.g. download page, yum, from source, etc.):

Describe the bug:
When creating an index pattern, if you have a field named _timestamp it will always be listed as an option for the time field for the index pattern, even if this field is already mapped as a non date type. Treating this field as a date causes issues in Discover, which fails to load index patterns that have this field due to date formatting being applied to a non-date data type, such as :

Cannot retrieve search results

error fetching [_timestamp]: Field [_timestamp] of type [float] doesn't support formats.

Steps to reproduce:

1. Create a document that has a _timestamp field:

PUT /timestamp_test/_doc
{
  "message": "doc 1",
  "@timestamp": "2021-12-20T18:27:23.436Z",
  "_timestamp": 1391759952.705
}

2. Verify that your _timestamp field has a numeric mapping with GET /timestamp_test/_mapping
3. Create an index pattern that captures the timestamp_test index, set @timestamp as the time field. Note that _timestamp was an option for the time field
4. Check the newly created index pattern, verify that _timestamp is correctly shown as a numeric field type
5. View the data in this index pattern in Discover
6. Get error that _timestamp doesn't support formats

Expected behavior:
If _timestamp isn't mapped as a date, we shouldn't try to to date things to it.

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[tsullivan]

I can reproduce this.

After creating the index pattern, the list of fields has the correct type for my _timestamp field:

But when running the search, the field is added to the query as a date_time field:

@elastic/kibana-app-services any idea why this { "field": "_timestamp", "format": "date_time" }, is in the request, instead of "format": "float" ? Is something hardcoded in the search API?

[tsullivan]

When looking at requests fired before the search request, there is a request for this endpoint:
<host>/api/index_patterns/_fields_for_wildcard?pattern=sparse*&meta_fields=_source&meta_fields=_id&meta_fields=_type&meta_fields=_index&meta_fields=_score.

The response contains this:

{
  "fields": [
    ....
    {
      "name": "_timestamp",
      "type": "date",
      "esTypes": [
        "float"
      ],
      "searchable": true,
      "aggregatable": true,
      "readFromDocValues": true,
      "metadata_field": false
    },
    ....
  ]
}