I really like how metadata works in the guides and grid examples, I searched in the API reference with “guide_x” instance but couldn’t find any results, are metadata instances documented somewhere else? I can write this documentation with the help of such reference (if it exists).
I do not exactly understand.
I you are looking for how ,synfig internaly speaking, grid metadata works, you can have a look to this commit who add grid color metadata to synfig.
Metadata panel shows you all the useful existing keys. You ca add mor personal keys but not useful for the program functionality. For example you can add the scene number or the author there, but they just will be stored in the file but not used by the application.
-G