Make metadata dataset type name formula available as classmethod.

We don't want to force a TaskDef to be constructed in order to get at
this.

python/lsst/pipe/base/pipeline.py

@@ -176,10 +176,26 @@ def metadataDatasetName(self) -> Optional[str]:
         metadata is not to be saved (`str`)
         """
         if self.config.saveMetadata:
-            return self.label + "_metadata"
+            return self.makeMetadataDatasetName(self.label)
         else:
             return None
 
+    @classmethod
+    def makeMetadataDatasetName(cls, label: str) -> str:
+        """Construct the name of the dataset type for metadata for a task.
+
+        Parameters
+        ----------
+        label : `str`
+            Label for the task within its pipeline.
+
+        Returns
+        -------
+        name : `str`
+            Name of the task's metadata dataset type.
+        """
+        return label + "_metadata"
+
     @property
     def logOutputDatasetName(self) -> Optional[str]:
         """Name of a dataset type for log output from this task, `None` if