Fix sort order on mixed date and date_nanos fields

[xeraa]

Kibana version: 7.3.0

Elasticsearch version: 7.3.0

Server OS version: RedHat

Browser version: Brave 0.68.131, Chromium: 76.0.3809.100

Browser OS version: Mac OS X

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

Describe the bug: When you are searching multiple indices and they are using a mix of date and date_nanos, the sort order is not correct. The reason is that the internal representation is a long since the start of the epoche for both, but because of the different precision this doesn't sort correctly. Elasticsearch has added a workaround for that in 7.2 with an option "numeric_type": "date_nanos" to cast everything to nanosecond precision.

Steps to reproduce:

1. Mapping

PUT timestamp-millis
{
  "mappings": {
    "properties": {
      "timestamp": {
        "type": "date"
      }
    }
  }
}

PUT timestamp-nanos
{
  "mappings": {
    "properties": {
      "timestamp": {
        "type": "date_nanos"
      }
    }
  }
}

2. Sample documents:

PUT timestamp-millis/_doc/1
{
  "timestamp": "2019-01-01T12:10:30.123Z"
}

PUT timestamp-millis/_doc/2
{
  "timestamp": "2019-01-01T12:10:30.124Z"
}

PUT timestamp-nanos/_doc/3
{
  "timestamp": "2019-01-01T12:10:30.123456789Z"
}

PUT timestamp-nanos/_doc/4
{
  "timestamp": "2019-01-01T12:10:30.123498765Z"
}

3. Sort in Discover (format in the index pattern is switched to Date Nanos):

Expected behavior: Sort correctly by using "numeric_type": "date_nanos" — probably only when the format of the index pattern is switched to Date Nanos.

Any additional context: I got curious after asked a question about it on Stack Overflow.

[elasticmachine]

Pinging @elastic/kibana-app

[timroes]

We should only apply that flag to sort if we're sorting over date_nanos I think, since it can actually cause failures in regular date fields if you're using dates outside a specific range, as states by the docs.

[kertal]

@xeraa Thx for the numeric_type hint!
@timroes agreed!
We also have to investigate Index Pattern creation for the mixed case aka date format melange