When generating metadata for media types of Dataset rows, optimize the case when all rows have same media type

Currently we set media type row by row, but with compacting disabled by default, we should consider that if all rows have same media type we instead set media type on ALL_ELEMENTS instead of row by row. Before we could rely on compacting to compact that.

We should add a benchmark as well. Current benchmark contains files with only one media type, we should keep one like that, but also add one with mixed (where this propose change will not be in effect).