[Maps] Tooltips with timestamp appear as epoch

[stratoula]

Kibana version:
7.11 ++

Describe the bug:
It seems that if I create a new Documents layer and use as a tooltip field a date, the date is not formatted but appears as epoch. I have tested it on 7.10.1 and it worked but it seems to be broken on 7.11

7.10.1

7.11.1

Steps to reproduce:

1. Create a new Documents Layer from kibana_sample_data_logs
2. Add @timestamp as the tooltip field
3. Hover over the points and see that the date is not formatted
   

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[thomasneirynck]

I can reproduce this on master. this is a regression.

[nreese]

Regression caused by #84377.

Maps lazy loads feature tooltip properties. Each time a tooltip is needed for a feature, a search request is generated to fetch the document. Prior to the change, the default search source behavior is used, which formats dates as date_time. After the change, the search source docvalue_fields is generated with the function getDocValueAndSourceFields. The problem is that getDocValueAndSourceFields specifically formats dates as epoch_millis so the value is a number that can be used for ordinal styling. The date field formatter does not work when dates are formatted as epoch_millis.

Prior to 7.11, request to fetch tooltip fields looked like

"docvalue_fields": [
    {
      "field": "@timestamp",
      "format": "date_time"
    }
  ],

After 7.11, request to fetch tooltip fields looks like

"docvalue_fields": [
        {
          "field": "@timestamp",
          "format": "epoch_millis"
        }
      ],

[nreese]

Formatting date's as epoch_millis hits the bug elasticsearch bug elastic/elasticsearch#50622 where dates formatted as epoch_millis are returned as strings instead of numbers. Since the value is string instead of a number, the field formatter can not format it.

The issue has been fixed before for feature labels by converting date epoch millis strings to numbers #63909.

We need to ensure the same thing is done for date fields used in tooltips