That's what happened to me, I had to take them off; yet TSA, only wanted me to un-hitch one of the front clasps, and said that suspenders (even with metal), don't set off the detector. Perhaps, they had their sensors set too "high" that day.
The difference is that mine don't fasten with metal, they have leather loops that button onto buttons inside my waistband which makes their removal more of a challenge.