MINOR: stats: rename proxy stats

MINOR: stats: rename proxy stats

This commit is the first one of a serie which adjust naming convention
for stats module. The objective is to remove ambiguity and better
reflect how stats are implemented, especially since the introduction of
stats module.

This patch renames elements related to proxies statistics. One of the
main change is to rename ST_F_* statistics indexes prefix with the new
name ST_I_PX_*. This remove the reference to field which represents
another concept in the stats module. In the same vein, global
stat_fields variable is renamed metrics_px.