Fabrice, looking at this again, I'm still not sure what you are trying to do. To me it looks like C++ log already supports mechanism (MDC) that anyone can use to add PID or any other information to the logger (and I have not seen any actual request yet to provide PID in C++ logger). Frankly I do not see reason to add specialized method (we have too many macros there already) for PID only. Could we instead extend documentation to say something like:
To log process ID information one could use MDC: e.g.:
(which has to be done after fork()) and add corresponding formatting code to the logger configuration.