as far as I know, Metabase doesn't support CSV files. Google DataStudio might be an option? but also no idea if it supports CSVs (I know its not open source, but still its free and online)
The expectation is that data is loaded into your database, which is reasonable. None of these tools are built to ingest data from multiple disparate sources and perform their own aggregations (like Tableau), but rather depend on locality of data on the database under query.
there is a csv driver[1] which allows you map a csv file on the metabase server, or providing a url of the csv. But I guess most people (including myself) would think of "upload csv file via UI" when it comes to csv support.
The expectation is that data is loaded into your database, which is reasonable. None of these tools are built to ingest data from multiple disparate sources and perform their own aggregations (like Tableau), but rather depend on locality of data on the database under query.