Of its 150kg body mass, about 1,5kg to 2kg are metals.
That's a 1-1.5% concentration, which is probably impossible for iron-age technology to refine, and even if it weren't, it competes very poorly with mineral ores. About 60% of a human body is water, so if this species has a similar amouunt, a thorough drying increases the concentration to 1.67-2.5%, but depending on the metal that's still quite low.
- Copper ores were available in the recent past with concentrations of 12%, and were likely much higher in the distant past (the bronze age needed easily accessible copper, after all). Modern ores can be as low as 0.6%, so your organisms would be viable for mining in the modern industrial era, but in the iron age it would probably be impossible.
- Nickel is harder to get hold of, and a 2% nickel ore would be pretty high grade. It is an important metal in the modern day, but wasn't really very useful until steel was invented. Compared to bog iron, a comparatively rich and fairly widely available source of iron which was exploited during the iron age, the paltry amounts of nickel available here just wouldn't be worth the effort even if they could be extracted. Which might not be possible with the technology of the day.
- Silver would be more interesting... the galena that would have provided silver in antiquity was sometimes as high as 1-2% (along with plenty of useful lead). It is remotely possible that lower yields of silver in your creatures might be worth exploiting, but in the absense of other high-yield useful metals it seems dubious. Also, lowering the rarity of silver will reduce its value, and it doesn't have many uses beyond art and currency, so you may choke your own market immediately.
- Cobalt wasn't refined as pure metal until the late 1700s. Depending on the form it has in the organism, some cobalt compounds might be useful for other purposes, such as colouring glass or ceramics. That was done as least as far back as the bronze age. The concentration, therefore, doesn't really matter.
So depending on the exact metals in your corpse-ore, it'll either be impossible or pointless for iron age people to extract, and somewhere between pointless and borderline for modern-day industry.
Also note that it is much more straightforward to refine regular high-grade mineral ores... iron bloomeries existed as long ago as 3000BCE, but your low-grade meat-based ore would almost certainly need much more sophisticated chemical and metallurgical technology in order to refine. And once that technology does exist, why would anyone bother using it on borderline mineral sources?
Instead, consider that the environment in which this organism lives sounds like an excellent place to set up an open-cast mine. So slaughter all the inconvenient wildlife, and get stuck into the useful minerals instead.
I thought about fire, but all those metals have different melting points, right?
Separating metals is somewhat easier than refining very low-grade ores. Fire and a bit of clever chemistry will sort you out eventually, but it is unlikely to be worthwhile in this situation.