main: return 404 in the root HTTP handler iff path is not "/"
main: return 404 in the root HTTP handler iff path is not "/"

Return 404 in the root HTTP handler iff the path is not "/" so that the
users would be able to access other end-points such as
"/debug/pprof/heap" and so on.

Signed-off-by: Giedrius Statkevičius <f1628506d066beb6229c6ca0fc8c69f80553b7c6@adform.com>

prometheus/graphite_exporter