Allow adding/editing of metadata when multiple nodes are selected

[DCON-b]

Metadata is really powerful, but when you select multiple nodes in the editor, the ability to add metadata disappears, meaning you can only add metadata to a single node at a time. This makes adding metadata to multiple nodes rather tedious.

The editor should support adding/editing metadata to nodes when multiple nodes are selected, similar to how editing any other property in the editor will edit the relevant property for each selected node.

[Calinou]

One difficulty is that multiple nodes may have different metadata keys available (and/or different values to them, but it's less of a concern here). Should we only display a metadata key when all selected nodes have the metadata key in question set, or should we display it as soon as one of the nodes have the metadata key set? In this case, what should changing the metadata do? Should it automatically create the metadata on the other nodes where it doesn't exist yet, or only set it if already existing?

[DCON-b]

Personally, the only feature I want is to be able to add metadata to multiple nodes at once- meaning that if I select multiple nodes and add metadata, it will be added to each node. (Since most of my use cases, I am simply checking for the existence of the key, and the value is mostly irrelevant. In cases where the value does matter, I usually set the values by code, which doesn't require metadata to already exist on the node, and as such, nothing is needed from the editor).

However, it would be nice to have the ability to edit multiple nodes metadata at once, and it makes sense from a UX perspective to make this work with editing metadata as well. I would expect that only keys which are unanimously shared by the selected nodes would show in the editor, similar to how non-metadata properties are displayed when selecting multiple nodes in the editor as of today.

To answer your questions directly:

Should we only display a metadata key when all selected nodes have the metadata key in question set

Yes.

or should we display it as soon as one of the nodes have the metadata key set? In this case, what should changing the metadata do?

In this case, I would expect:

it automatically creates the metadata on the other nodes where it doesn't exist yet