MEDIUM: promex: switch to using stat_cols_px for front/back/server metrics

MEDIUM: promex: switch to using stat_cols_px for front/back/server metrics

Now the stat_cols_px array contains all info that-prometheus requires
stop using the promex_st_metrics array that contains redundant infos.

As for ("MEDIUM: promex: switch to using stat_cols_info for global
metrics"), initial goal was to completely get rid of promex_st_metrics
array, but it turns out it is still required but only for the name
mapping part now. So in this commit we change it from complex structure
array (with redundant info) to a simple ist array with the
metric id:promex name mapping. If a metric name is not defined there, then
promex ignores it.