Gophercloud currently only supports setting tags while creating a server or getting tags from an existing server but there is no support to perform updates on server tags.

For: terraform-provider-openstack#722

Docs: https://docs.openstack.org/api-ref/compute/?expanded=#server-tags-servers-tags

• List tags Compute V2: add tags extension with List call #1678
• Check tag existence Compute V2: add tags Check call #1679
• Add single tag Compute V2: implement Add method for Tags #1695
• Replace all tags Compute V2: add Replace method for Tags #1694
• Delete a single tag Compute V2: add Delete method for Tags #1699
• Delete all tags Compute V2: add DeleteAll method for Tags #1700
• Tags in server body Compute V2: allow to read Tags from Server body #1734
• List / filter servers by tags Compute V2: implement server tags filtering support #1759