log faster when log level is not high enough

[UnixJunkie]

dbuenzli's log library (https://github.com/dbuenzli/logs) does this:

Logs.err (fun m -> m "invalid kv (%a,%a)" pp_key k pp_val v);

apparently it allows to skip the formatting of things which are not going to be printed

[UnixJunkie]

maybe we need to extend the current interface in order to support that

[UnixJunkie]

There is a discussion about such tricks there: https://discuss.ocaml.org/t/format-kprintf-usage/1396/18