The magnet is there specifically to collect any metal particles, and the manual instructs you to clean the plug at every oil change, which means a little bit is considered normal. But if you get relatively big shavings and/or lots of them, at every oil change, then maybe something is not right. Difficult to judge without knowing what you consider "a decent amount of metal shavings". I personally don't recall seeing anything out of the ordinary on the 899 (in terms of metal shavings on the plug), compared to other bikes. If you suspect something is not right, ride for 500-1000 miles, drain the oil and check the plug (if you do it very carefully in a clean place with a clean drain pan, you can even re-use the oil).