[Lens] Hierarchical header for splitted columns in data table

[flash1293]

If columns are splitted multiple times in Lens, the values are repeated

By having a two-row header with colspan it would be possible to group the columns, making it much easier to read the table:

It's omitted in the screenshot, but there can be a separate layer to this hierarchy which is the metric (as it's possible to use multiple metrics in the table). If we are using multiple metrics, they can simply be shown as the lowest level in the hierarchy.

If there is just a single metric, we could do multiple things:

• also omit this level as it isn't adding useful information in most cases
• make it configurable whether this level is shown
• show the metric as the top level in the hierarchy

[elasticmachine]

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

[ghudgins]

the first person I got to review the transposed row-to-column feature in 7.13 wanted this

[timductive]

Closing for now until we have more customer requests for this as it is a high effort