MINOR: promex: get rid of promex_global_metric array

MINOR: promex: get rid of promex_global_metric array

In this patch we pursue the work started in 1adc796 ("MEDIUM: promex:
switch to using stat_cols_info for global 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
promex_global_metric 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.