MINOR: stats: hide some columns in output

MINOR: stats: hide some columns in output

Metric style stats can be automatically calculate since the introduction
of metric_generate() when using "struct stat_col" as input. This would
allow to centralize statistics generation. However, some stats are not
outputted under specific condition. For example, health check failures
on a server are only reported if checks are active.

To support this, define a new function metric_hide(). It is called by
metric_generate(). If true, it will skip metric calcuation and return an
empty field value instead. This allows to define "stat_col" metrics and
calculate them with metric_generate() but hiding them under certain
circumstances.