The Metrea crank will first of all need a different bottom bracket, which is no problem. What you need to worry about is frame clearance. Depending on how your bike is set up now, it may put its single 42t ring where your 34t is currently, a big jump. Whether the frame will be able accommodate it without rub isn't something a forum can help you with.
To figure it out, you'd pull its chainline measurement off si.shimano.com and compare that to a firsthand chainline measurement of where your current 42 from your triple is falling. From that comparison you may be able to easily see sure, no problem, or you may find that you have to look more closely. If the latter, use a ruler or caliper to get a sense of whether it could go in as much as it's going to and still give about 3mm of clearance minimum, usually considered the smallest reasonable number to accomodate flex.
CS-5800 will not go on FH-RM35. No 11-speed road cassette will.