Edit meta tags per entity via field or entity property? Is there a way in Drupal 8 to override the global meta tags at the entity (node) level when adding or editing content? I thought the option previously existed in 7.x, but I am not seeing it in 8.x.

If not, is there a way to alter the meta tags when preprocessing a node to inject them based on node fields in code?
Yes, there is.

All you need to do is add the Meta tags field to the node types where you want to have this.