These metrics are fairly essential if you want to be sure your users are actually receiving your content, and what content they are relating to in order to improve your content.
Since providing relevant content which actually makes it into your users’ Inboxes is the whole point of a newsletter, it seems like ensuring that is actually happening is a core part of the service.
But I am not a lawyer, and I am certainly no expert on GDPR.
Simply because the industry has molded the competitive landscape in such a way that tracking can be deemed "essential" does not, in my mind, make their behavior and actions immune from consent. But, I don't doubt that they could make that argument and win.
Maybe one day a prosecutor will use a database schema as evidence of GDPR violations...