It's just a top cap, fortunately it's replaceable, I've got to replace one of mine as it's also slightly cracked and rattles. If you pull the lever fully, you'll see a small screw that holds it in place, it's just a case of unscrewing that and putting a new one on.
I'm assuming it's R7000 105 - but is it rim brakes or hydraulic disc - there are slight differences between the part numbers you need.