Metric to support a counter attribute

Frequently I have a `dataclass` that: stores a histogram of data, counting occurrences of some enumeration

```python
@ddataclass(frozen=True)
class Counts(Metric["Counts"]):
    identifier: str
    counter: Counter[SomeEnum]
```

where `SomeEnum` is an enumeration.

Since the definition of the enumeration is fixed, I would like a column per possible enumeration value.