[Lens] unable to add last value of date/string field

[tsullivan]

Kibana version: 7.14.0-bc1
Original install method (e.g. download page, yum, from source, etc.): docker
Describe the bug:
Input validation prevents the user from configuring a table in Lens from showing the last value of a date field.

Steps to reproduce:

1. Install the ecommerce sample dataset
2. Go to Visualize Library > Create and choose Lens
3. Change the chart type to table
4. Drag the email field in the table. Lens creates a "Top values of email" table:
   
5. Click the "Count of records" metric
6. Choose the "last value" function and look for date fields.
   

The date fields are in light gray text. When you click on one of them, the function automatically changes to "Unique count".

The same limitation happens with Maximum and Minimum. Looks like that is already filed as an issue: #95489

Expected behavior:
"Last value of date" should be supported.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[ghudgins]

+1

Getting the latest timestamp from the newest event would be nice. Timestamp type metric field. "Latest value" plus timestamp does not work, and there is no date metric field.

[m-adams]

+1 for date but also string should be supported for last state type of use cases.