Which ETL tool you are using? ETL tool provides you output in database form & you can connect Tableau with the database output. If you are using Talend then you can get output by using tTableauExtractOutput(which provides you a database) & after that you have to extract the data from this file(I think in .tde format). You can also use R for direct ETL but for that you have to write different scripts in Tableau. I do not think that this is an easy process.
So, just focus on the output of your ETL process.