Add GroupByMetricReference (#292)

### Description
A cleanup item from metrics as dimensions. This class is currently in
MetricFlow, but it should live here.

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guide](https://github.com/dbt-labs/dbt-semantic-interfaces/blob/main/CONTRIBUTING.md)
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- [x] This PR includes tests, or tests are not required/relevant for
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dbt_semantic_interfaces/references.py

@@ -56,6 +56,16 @@ class MetricReference(ElementReference):  # noqa: D
     pass
 
 
+@dataclass(frozen=True)
+class GroupByMetricReference(LinkableElementReference):
+    """Represents a group by metric.
+
+    Different from MetricReference because it inherits linkable element attributes.
+    """
+
+    pass
+
+
 class ModelReference(SerializableDataclass):
     """A reference to something in the model.