Since [redesign](8553c77733)
of fivegs_smffunction_sm_sessionnbr gauge, the metric doesn't
expose some decrements. The decreasing of gauge had been
moved out of function stats_remove_smf_session.
It should be decreased every time stats_remove_smf_session
is called, but this particular case is easily reproducible
by killing UPF while the session is established.
12c0310328