feat: add additional_blob_attributes to upload_many_from_filenames

[andrewsg]

Fixes #996 🦕

[cojenco]

Is there a reason not to use blob._patch_property() that is broadly used in the library? Blob property changes are propagated through blob._get_writable_metadata and blob._get_upload_arguments

[andrewsg]

I did consider that but decided there was no need to use a private method on the blob class when the typical public interface of attribute assignment would do the same thing.

[cojenco]

I'm curious how the blob properties are actually picked up in blob._changes. I thought we're relying on blob._changes to get writable metadata for the upload arguments

python-storage/google/cloud/storage/blob.py

Lines 1708 to 1713 in c5a983d

	object_metadata = {"name": self.name}
	for key in self._changes:
	if key in _WRITABLE_FIELDS:
	object_metadata[key] = self._properties[key]
	return object_metadata

[andrewsg]

In the _PropertyMixin, all the fields set as a _scalar_property have a custom setter that updates _changes. This solution using setattr will also activate that.

[cojenco]

Ah I see, that makes sense, thanks! One last nit on the naming, maybe consider additional_blob_properties for consistency? Property is used in the official docs and in the lib.

[andrewsg]

Haha, that was actually the name in my first revision.

I changed it to "attributes" because it's not only _scalar_property but any attribute that we can set with setattr. Also, since we're using setattr the nomenclature matches.

[andrewsg]

I'm going to add a system test to this before merging, working on it now.

[cojenco]

LGTM, thanks!