so far, I have seen (mine being one of them) at least two threads on the bike having problems being stuck (and then unstuck) in third gear. Mine seems to be working now. Several years ago a national tranmission chain used to put metal filings in the tranmission fluid drained off to get people to rebuild their transmission.Has any aspect of the bikes performance altered. (gear changes, engine noises, clutch sticking etc)
If the answer is no to the above I wouldn't think you have anything to worry about. The oil filter is doing its job.
My geuss is that the metal has come from that horrible clunky gearbox that the 1200's are cursed with. (I ride a 87 aspy)
You did very well to spot the metal, unless grains of sand are bigger on your side of the world.