Expand marker schema to allow dynamic label

[squelart]

Currently the schema for marker tooltip rows only allows label: value lines where value may be static or dynamic (i.e., taken from the marker data payload), but the label can only be a fixed string.

It would be useful in some cases to allow that label to also be dynamic. This was suggested here, where some code wants to display key+value pairs coming from an API, where both keys and values are only known at runtime.

This would require some work on the backend as well. When someone can work on this here, please file a corresponding bug there.

[parttimenerd]

This is a bad idea for performance reasons: It would hinder us from storing the marker data in lists rather than in objects. And also: It's more a hackish fix for the deeper problem that formatting of markers is too limited. I proposed more formatting options in #4201, which includes a table type that might be suitable for your problem.