The metal piece needs to come out. It is from the old weather strip. Be careful removing it -- there is a danger of cracking the windshield.
The metal is a U shaped piece. I used a metal pick to grab the metal channel so I could pull it backwards off the windshield. The hardest part was getting it started. Once you have a few inches free then rest comes out easy.
I had a hard time starting it from one end, so I moved to the other and it came out much easier.
Make sure to really vacuum/clean the area before trying to reinstall. There is a gap under the top edge of the windshield where those rubber remnants can build up and will prevent the new one from going in.
When installing the new one use a LOT of Windex on the glass and roof as a lubricant. This will allow you to "slide" the piece back and forth to get a good fit.
FCP has a great video showing the process.