Here is a close up of the oil filter element. Inspect the pleats of the element carefully. What you are looking for are shavings which are very bad, these using come from dying thrust bearings or if you spun a bearing and its pieces of bearing material, the steel bearing backing and your block or rods. Usually, this is accompanied by a big drop in oil pressure and loud knocking noises coming from your engine’s bottom end. Usually, most of the shavings are magnetic. Sometimes shavings also come from the valvetrain and are from the cam and cam followers. This is usually accompanied by ticking or knocking noises coming from the top of the engine.
The other thing you are looking for is flat flakes shaped metallic particles. These are usually not magnetic. These are coming from the engines bearings and are not a good sign either.
The end of the oil filter case is where a lot of metal debris accumulate. The oil flow stagnates in this area and a lot of the metal in the oil ends up deposited here, usually, most of the metal debris are found here, often more than what ends up on the filter element. I usually pour this small amount of oil onto a clean paper towel and inspect it for metallic debris then inspect and wipe out the case with a clean towel looking for any metal or other material on the towel. I also usually look at the anti-drain back valve spring that sits in the bottom of the case to see if there is any material on it as well.
If you see these sorts of particles in the filter, it is probably a good idea to tear down your motor and find their source as there presence means that something bad has happened or is in the process of happening and if the motor is torn down and repaired now, a catastrophic and much more expensive failure can be avoided.