Can putting my Apple Pencil against a metal surface make it loose power?

No. Placing your Apple Pencil adjacent to, or on, a metallic surface will not cause the Pencil battery to lose charge.

NB: If you have a second generation Apple Pencil, and place the Pencil on a ferrous metal surface (such a steel), the Pencil magnet will hold the Pencil on the metal surface.

While it's true that it won't lose charge being on a metallic surface such as the metal leg of a desk, it IS true that it can render your Apple Pencil Gen 2 completely unusable. This happened to me. I recently had to get a new one because it was unfixable. It wouldn't pair with any devices, or work at all. I recommend keeping it in its box when not using it. Placing on your iPads side to charge is fine, too, however that can fall off quite easily in transit. Moral of the story: DO NOT just think you can place your magnetic Apple Pencil Gen 2 on any metallic surface simply because you don't notice a drain in battery. FAAFO!