fix(task): crash in key check when metadata already copied into the c…

…ontainer

This caused the pipeline to crash when the output container already had
the version or config tags present. This happens when `attrs_from` is
used to transfer over metadata.

draco/core/task.py

@@ -401,10 +401,9 @@ def _save_output(self, output):
             # add metadata to output
             metadata = {"versions": self.versions, "config": self.pipeline_config}
             for key, value in metadata.items():
-                if key in output.attrs:
-                    raise RuntimeError(
-                        "Can't write {} to output: it already exists.".format(key)
-                    )
+                # NOTE: this will overwrite any existing metadata keys. This is probably the right
+                # thing to do for now, but really we should be using the `history` functionality for
+                # this.
                 output.attrs[key] = value
 
             # Create a tag for the output file name