Thanks- I figured out what that error was all about.
I'm not trying to edit any metadata at all: that's all done. I just want to publish my centerlines to AGOL and have the metadata that I painstakingly created be available to AGOL users.
Our Egdb features have attribute rules associated with them, so prior to publishing them, I write them from the EGDB to a FGDB and publish from there. Prior to publishing, I can view the full metadata from the fgdb feature class in a Catalog View; it is complete all the way to attribute descriptions.
If I perform a 'save as' XML file from the Meta Data group in the Catalog Group on that fgdb feature class, I can read that xml file in catalog by simply navigating to it, and it appears complete.
From a map in ArcGIS Pro, with the Share tab I share the fgdb feature class as a Weblayer to my AGOL account. In AGOL, I click on the item's metadata button and select 'view' and I'm given this window:
After clicking save and view, this is what is returned:
I then use the overwrite option and use the local xml that I created with the save as step. Save and view then returns a bit more metadata in a browser window.
What exactly is the big white window that opens? Is that considered a viewer or editor? Depending on the the tab I select, certain items are on or off, but if I search long enough I can dig down deep enough to get to the contact info and attribute descriptions and other details that I expect to see.
That should just about do it....