Move from manifest.metrics.get to manifest.expect in metric helpers

Previously with `manifest.metrics.get` we were just skipping when `None`
was returned. Getting `None` back was expected in that `parent_unique_id`s
that didn't belong to metrics should return `None` when calling
`manifest.metrics.get`, and these are fine to skip. However, there's
an edgecase where a `parent_unique_id` is supposed to be a metric, but
isn't found, thus returning `None`. How likely this edge case could
get hit, I'm not sure, but it's a possible edge case. Using `manifest.metrics.get`
it we can't actually tell if we're in the edge case or not. By moving
to `manifest.expect` we get the error handling built in, and the only
trade off is that we need to change our conditional to skip returned
nodes that aren't metrics.

core/dbt/contracts/graph/metrics.py

@@ -41,9 +41,9 @@ def parent_metrics(cls, metric_node: Metric, manifest: Manifest) -> Iterator[Met
         yield metric_node
 
         for parent_unique_id in metric_node.depends_on.nodes:
-            metric = manifest.metrics.get(parent_unique_id)
-            if metric is not None:
-                yield from cls.parent_metrics(metric, manifest)
+            node = manifest.expect(parent_unique_id)
+            if isinstance(node, Metric):
+                yield from cls.parent_metrics(node, manifest)
 
     @classmethod
     def parent_metrics_names(cls, metric_node: Metric, manifest: Manifest) -> Iterator[str]:
@@ -63,9 +63,9 @@ def reverse_dag_parsing(
             yield {metric_node.name: metric_depth_count}
 
             for parent_unique_id in metric_node.depends_on.nodes:
-                metric = manifest.metrics.get(parent_unique_id)
-                if metric is not None:
-                    yield from cls.reverse_dag_parsing(metric, manifest, metric_depth_count + 1)
+                node = manifest.expect(parent_unique_id)
+                if isinstance(node, Metric):
+                    yield from cls.reverse_dag_parsing(node, manifest, metric_depth_count + 1)
 
     def full_metric_dependency(self):
         """Returns a unique list of all upstream metric names."""