MINOR: stats: update ambiguous "metrics" naming to "stat_cols"

MINOR: stats: update ambiguous "metrics" naming to "stat_cols"

The name "metrics" was chosen to represent the various list of haproxy
exposed statistics. However, it is deemed as ambiguous as some stats are
indeed metric in the true sense, but some are not, as highlighted by
various "enum field_origin" values.

Replace it by the new name "stat_cols" for statistic columns. Along with
the already existing notion of stat lines it should better reflect its
purpose.