This should allow us to gather a bit more information about errors that
we encounter when creating UPnP mappings. Since we don't have a
"LabelMap" construction for clientmetrics, do what sockstats does and
lazily register a new metric when we see a new code.
Updates #9343
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ibb5aadd6138beb58721f98123debcc7273b611ba