MEDIUM: contrib/prometheus-exporter: Rework matrices defining Promex metrics

MEDIUM: contrib/prometheus-exporter: Rework matrices defining Promex metrics

The global and stats matrices are replaced by a simpler ones. Now we have
only 2 arrays of prometheus metrics. Their flags are used to match on the
entity type. This simplify a bit the metrics definition. For now, labels and
descriptions are still outside of these arrays, because the labels must be
reworked to be more dynamic and the descrptions must be replaced by stats
ones as far as possible.