Creatures that evolved in metal-rich environments would be immune
Evolution could take care of the toxicity problems. Metal toxicity is a characteristic of many different metals, and the root cause for why a metal is poisonous varies widely between metals. For example with arsenic:
Arsenic interferes with cellular longevity by allosteric inhibition of
an essential metabolic enzyme pyruvate dehydrogenase (PDH) complex,
which catalyzes the oxidation of pyruvate to acetyl-CoA by NAD+. With
the enzyme inhibited, the energy system of the cell is disrupted
resulting in cellular apoptosis. Biochemically, arsenic prevents use
of thiamine resulting in a clinical picture resembling thiamine
Arsenic interacts with specific enzymes in a specific way to disrupt them from doing their biological job. The solution is for organisms to evolve specific methods for resolving whatever biochemical problem the metal causes. For arsenic, an animal which evolved so that that allosteric sites on the PDH complex enzyme are not receptive to arsenic. The biochemical reaction chain is broken, and now arsenic is not any more problematic to us than eating iron (well...there may be other problems arsenic causes, this is just the one that kills you fastest).
Humans are immune to the metals common in the environment
In the Earth's biosystems, certain metals are present in certain quantities. These metals are needed for nutrition, iron, magnesium, potassium, etc. The amount of iron or aluminum, two common metals, found in organic matter will not poison an organism on Earth, since all organisms on Earth have had to deal with these common metals for billions of years.
If you planet has a different geo-chemistry, or different elemental abundances in the crust, the creatures of your planet, since the inception of life, will evolve a resistance to the metals commonly encountered. Therefore, you can assume that any creature on a heavy-metal-heavy planet would be immune to the commonly encountered heavy metals.