Ah, scratch that, you can't do it with top hit metric (that is the metric aggregation I choose). It may have only worked in my example because of a fluke, or because I didn't have a time field. The Top Hit metric chooses the most recent value in the document, which may be an empty string. It's almost as if you want Max metric but on a string field, not a numeric field, but that doesn't exist.
Unfortunately I'm not aware of a way to achieve what you are looking for. I think your best bet may be to try and combine the rows at index time, so instead of indexing a separate document for each field, you index a single document with all the fields filled in.
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