Artisan’s mettle is incredibly useful, and one of the best things you can do for 10.2 is to make sure you stock up on it.
The main reason Artisan’s mettle is incredibly value for 10.2, is that it allows you to guarantee quality 3 for a bunch of different crafts by using Illustrious Insight. This can dramatically increase the expected profit. Couple this with the chance that a lot of items go up in price with the increase in demand that happens around patch launches and you are in position.
If you have enchanting you want to be able to absolutely crank out max quality devotion enchants, and with insight you can!
How do you get mettle?
You get mettle whenever you earn profession knowledge. So the main way to generate it is simply to stay on top of your knowledge gains. You also get some weekly mettle from the Artisan’s consortium based on your reputation. So for any characters crafting items that can only reach max quality with mettle you will want to farm your knowledge.
The absolute golden egg is the Dragon Shards of knowledge. They give a whopping 50 mettle, which is enough for an insight craft by itself. These can drop from any content type, so prioritize earning these on the right character.
What professions use mettle well?
You need professions where you can’t reach max quality without mettle, but you can with. Enchanting is my main pick, as you can guarantee quality 3 on weapon enchants with mettle if you also use the elemental shatter. This requires a ton of knowledge though. Using it to guarantee a specific quality on crafting orders for a higher fee can also be a method.