[Lens] Terms aggregation keeps the numeric formatter for non-numeric data

[wylieconlon]

This is a bug on master and 7.10. The user-facing effect of the bug is that labels for their data might not be correct.

Steps to reproduce:

1. Create an initial XY chart
2. Drag a numeric field onto the horizontal axis, which will create an Intervals aggregation
3. Switch the horizontal axis to Top Values
4. Choose a number formatter, anything but Default
5. Switch the field to any non-numeric field, and the chart will show incorrect data because it's not numeric
6. To work around the bug, switch back to the Default formatter.

The main source of this bug is that the state of the terms aggregation is not being fully updated when the field is switched. If the initial field is a number, the terms aggregation will report that it is always a number. So the solution is to write a more complete onFieldChange function for the Terms aggregation.

This PR might have merge conflicts due to my work on #82550, so I will work on both. If needed this will end up as two PRs.

[elasticmachine]

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