MINOR: stats: implement automatic metric generation from stat_col

MINOR: stats: implement automatic metric generation from stat_col

This commit is a direct follow-up of the previous one which define a new
type "struct stat_col" to fully define a statistic entry.

Define a new function metric_generate(). For metrics statistics, it is
able to automatically calculate a stat value field for "offsets" from
"struct stat_col". Use it in stats_fill_*_stats() functions. Maintain a
fallback to previously used switch-case for old-style statistics.

This commit does not introduce functional change as currently no
statistic is defined as "struct stat_col". This will be the subject of a
future commit.