Update the metric.expr to be populated correctly according to DSI rules

Author: WilliamDee

core/dbt/parser/schema_yaml_readers.py

@@ -643,7 +643,9 @@ def _create_metric(
             name=measure.name,
             label=measure.label or measure.name,
             type="simple",
-            type_params=UnparsedMetricTypeParams(measure=measure.name, expr=measure.name),
+            type_params=UnparsedMetricTypeParams(
+                measure=measure.name, expr=measure.expr or measure.name
+            ),
             description=measure.description or f"Metric created from measure {measure.name}",
             config=config,
         )