MINOR: promex: get rid of promex_st_metrics array

MINOR: promex: get rid of promex_st_metrics array

In this patch we pursue the work started in a5aadbd ("MEDIUM: promex:
switch to using stat_cols_px for front/back/server metrics"):

Indeed, while having ".promex_name" info in stat_cols_info generic array
was confusing, Willy suggested that we have ".alt_name" which stays
generic and may be considered by alternative exporters for metric naming.
For now, only promex exporter will make use of it.

Thanks to this, it allows us to completely get rid of the
stat_cols_px array. The other main benefit is that it will be much harder
to overlook promex metric definition now because .alt_name has more
visibility in the main metric array rather than in an addon file.