[Lens] Allow use of top metric for last value

[flash1293]

Right now Lens is using top hits to power the last value function. However, this has a major downside - it's not possible to sort a terms agg by a top hits agg (it's also less efficient).

By using top metrics instead of top hits, we could use sorting and also provide better performance, but it has a downside as well - array values are not supported, something we offer in the current version of last value.

Previous discussion: #98733

Absent a better solution, to offer the best possible feature, we should offer a choice in the UI:

• Implement top metric as an agg type so it's possible to use it
• By default, compile last value to top metric and mark it as sortable
• In the last value dimension configuration provide a switch "show array values" (wording tbd), to switch it over to top hits (set to true by default for existing visualizations)
• If the selected field is a scripted field, do not allow to disable "show array values" (always use top hits)
• If possible (top metrics is used) include the last value dimension in the regular top values ranking logic

[elasticmachine]

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

[flash1293]

@Dosant Is there an issue to point to for the planned "top metric" work on the app services side?

[Dosant]

@flash1293 here it is: #124535