Do we need this?

A motivating example would be to have events such as session-joined and the like.

Alternatively it's true that structured logging can be handled as stringly-typed regular logs. Some type information and state would be lost here. For example logging a shared_ptr can't possibly influence the ref count.

High-velocity events and profiling-type information can (and likely should) be handled by dedicated intrusive profilers like https://github.com/iboB/ibp

So far I'm not sure that structured logging is a feature worth pursuing.