Then which variable should I use to merge data to metro level?
Assuming, based on your prior question, that you’d like to use 2015 public use microdata, then MET2013 is the only option available.
The Census does not identify metro areas in public use ACS microdata. The only identified geographic areas smaller than states are PUMAs, and IPUMS derives MET2013 from the available PUMA info, which allows us to identify only a limited set of metro areas. Unfortunately, it is not possible to identify _all_ metro areas in 2015 PUMS.
If by chance the metro area data you need has been published in ACS Summary File tables, you can obtain that data through NHGIS, another MPC/IPUMS site.