BUG/MINOR: stats: Continue to fill frontend stats on unimplemented metric

BUG/MINOR: stats: Continue to fill frontend stats on unimplemented metric

A regression was introduced by the commit 0ef54397b ("MEDIUM: stats: allow
to select one field in `stats_fill_fe_stats`"). stats_fill_fe_stats()
function fails on unimplemented metrics for frontends. However, not all
stats metrics are used by frontends. For instance ST_F_QCUR. As a
consequence, the frontends stats are always skipped.

To fix the bug, we just skip unimplemented metric for frontends. An error is
triggered only if a specific field is given and is unimplemented.

No backport is needed except if the above commit is backported.