Like carbs, heavy metals aren't entirely "bad for you." This article is a brief on the what & why of heavy metals, and a little on how they're communicated on your food labels.
(1) Metallic elements,
(2) Of a high atomic weight; a density at least 5 times greater than that of water.
There lies the technical definition of a heavy metal. 
Heavy metals are perceived with an assumption that heaviness and toxicity are inter-related.
However, heavy metals can also be essential nutrients – required for various biochemical and physiological functions. An inadequate supply would result in a variety of deficiency diseases or syndromes, or at least, a less than optimal quality of health. 
Some examples of essential heavy metals include:
Some metals are essential to the extent of required labeling on Nutrition Facts Panels (NFP):
(Note: NFP compliance effective 2020)
You can click here for a description of function, dietary reference intakes (DRI values), food sources, and adverse effects of excessive consumption unique to these elements.
Elaborating on the topic of elements, below is an image of the Periodic Table. Metals, metalloids, and nonmetals have been color-coded for ease in differentiating element classification. The yellow-shaded portions represent those elements (regardless of metal, metalloid, or nonmetal) that are naturally-occurring in food products, some falling under "ash content," more specifically, "minerals" on a Nutrition Facts Panel.
Figure 1. Periodic Table of Elements, The Elements in Food
Many (of the yellow-shaded elements) are optional for the manufacturer to list, but all are identified on the FNB's Dietary Reference Intake resource. The elements: Boron, Silicon, Nickel, Vinadium, and Arsenic do not have RDA nor AI values. These values are not determinable due to lack of data of adverse effects and concern with regard to lack of ability to handle excess amounts. However, it is noted the source of intake should be from food only to prevent high levels of intake . 
How much is too much?
All elements, at a certain dose, are toxic. Toxicity is dependent upon a variety of factors, including gender, age, weight, life stage, and the specific state of one’s health and/or conditions.
As it pertains to the food we eat, the FDA has established four sets of Daily Values (DVs) for labeling of foods and dietary supplements:
(1) Adults and children 4 years and older
(2) Children 1 through 3 years
(3) Infants 1 through 12 months
(4) Pregnant and lactating women
In establishing these DVs, the FDA selects the highest RDA value within each of these four age and condition groups. The values are established by the National Academies of Sciences, Engineering and Medicine's Food and Nutrition Board (FNB) – a responsibility previously carried out by the Institute of Medicine (IOM).
To view the FDA’s Daily Values unique to each micronutrient, in some cases, essential metals (even heavy metals), click here.
A lot goes into creating a Daily Value (DV).
But first, some terminology:
- DV = Daily Value
The Daily Value (DV) offers a single value for each nutrient - respective of the four age and condition groups (mentioned above). With that said, it is sometimes similar to one’s RDA for that nutrient. It was created to provide the consumer with an understanding of how much a food product contributes to their overall daily needs of a specific nutrient (in % form). 
- DRI = Dietary Reference Intakes
DRI encompasses both RDA and AI of over 40 nutrient substances, as established by the FNB. DRI is intended to serve as a guide for good nutrition and provide scientific basis for the development of food guidelines in the USA and Canada. 
- RDA = Recommended Dietary Allowances
RDA is the average daily dietary intake level. Specifically, it is the level sufficient to meet the nutrient requirements of nearly all (97-98 percent) healthy individuals in a group. It is calculated from an Estimated Average Requirement (EAR). 
- AI = Adequate Intake
If sufficient scientific evidence is not available to establish an EAR, and thus calculate an RDA, an AI is usually developed. 
- FNB = National Academies of Sciences, Engineering and Medicine's Food and Nutrition Board
- IOM = Institute of Medicine of the National Academy of Sciences
How DV Works (in brief).
1. DVs are reported in % form on a Nutrition or Supplement Facts Label. In some cases, the amount (of each nutrient) by weight is provided (for example, g, mg, or IU units).
2. The DV limits are set by the FDA, where the % is a representation of how much (of that nutrient) is present, in comparison to the amount recommended daily. The DV will vary by category: (1) Adults and children 4 years and older; (2) Children 1 through 3 years; (3) Infants 1 through 12 months; (4) Pregnant and lactating women.
3. The DVs are based on the RDA value. You can view them here.
Note: While the DV is based on the RDA value, RDA (and AI values) have been replaced by DRI (encompassing both RDA & AI) since the mid-1990’s.
4. The DRI values are determined by the FNB. Note: Previously, the IOM carried out this responsibility. As of July 1, 2015, the FNB is the responsible organization.
5. The FNB determines the DRI value unique to the following categories: Infants, Children, Males (9 years