Adding `Coord._set_metadata` completely negates the performance benefit. :expressionless:

Plus there's still the question of how to handle attributes which are specific to DimCoord (i.e. currently just `circular`). One can't just override `_set_metadata` because it's used by `Coord.__init__` which has no knowledge of the extra attributes. Anyway ... it's all hypothetical ...there's no point in the PR if there's no benefit :wink: