option A: lib has two features
- `defmt-impl`: provides `defmt::Format` impls
- `defmt-logging`: makes the lib do logging with `debug!`, `panic!` etc

option B: lib has one feature
- `defmt`: provides `defmt::Format` impls AND makes the lib do logging with `debug!`, `panic!` etc

there's no advantage of option A over option B. If you want the impls but no logging, you can achieve it with option B by shutting off the lib's logging with DEFMT_LOG.

and there's some disadvantages (extra complexity)