pceplib: add json string option to subgroup counter

pceplib: add json string option to subgroup counter

The created counters in pceplib library are structures with
a string attribute which is used for further display. This
string information is not formatted for json output.

Add a second option in the create_subgroup_counter() creation
API to provide the json attribute output.

Create a json naming compatible with caml format for each
subgroup counter used.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>