MINOR: stats: factor the proxy vs scope check into its own function

MINOR: stats: factor the proxy vs scope check into its own function

We'll need to check for matching between a proxy and a stats scope at
a few more places, so let's first move the code out of its current
function (stats_dump_proxy_to_buffer) into its own function
(stats_proxy_in_scope). At this point it doesn't change anything.
Note that this will have to be backported as this will be used with
a subsequent fix.