BUG/MEDIUM: promex: Fix dump of extra counters

BUG/MEDIUM: promex: Fix dump of extra counters

When extra counters are dumped for an entity (frontend, backend, server or
listener), there is a filter on capabilities. Some extra counters are not
available for all entities and must be ignored. However, when this was
performed, the field number, used as an index to dump the metric value, was
still incremented while it should not and leads to an overflow or a stats
mix-up.

This patch must be backported to 3.0.