This commit is part of a series to align counters usage between
frontends/listeners on one side and backends/servers on the other.
On frontend side, "stot" is the total count of sessions for both proxies
and listeners. For proxies, fe_counters <cum_sess> is correctely used.
The bug is on listeners where <cum_conn> value is returned, which
instead indicates a number of connection. This commit fixes this by
returning <cum_sess> counter value for "stot" metric.
Along this fixes, use the opportunity to report "conn_tot" for listeners
using <cum_conn> value, as for frontend proxies.
This commit fixes a bug but must not be backported as stats output is
changed.