I feel what is missing here is message templates [1] - the logging API should permit the key-value pairs to be substituted into a template which results in a human-readable message, while preserving the KV data separately. Take a hash of the template and add it as a KV pair so that messages of the same type can be easily filtered.
Loggers are just façade objects on Handlers, which is an interface.
It’s really designed to be a minimum necessary package to allow interop (via handlers) and a baseline of standalone usability (via loggers). The stdlib only provides a text and a json handler, not even a no-op handler which I think is sorely neededor a multi handler which I think would make a lot of sense.
But nothing precludes you publishing a messagetemplates handler, or whatever else you may want.
[1] https://messagetemplates.org