Addresses route-prefix bug described in prometheus#190

Pass in the prefix to the Static handler such that it can be removed
from the Path looked up to serve files relative to the FileSystem's
root.

handler/misc.go

@@ -63,9 +63,13 @@ func Ready(ms storage.MetricStore) http.Handler {
 // Static serves the static files from the provided http.FileSystem.
 //
 // The returned handler is already instrumented for Prometheus.
-func Static(root http.FileSystem) http.Handler {
+func Static(root http.FileSystem, prefix string) http.Handler {
+	handler := http.FileServer(root)
 	return promhttp.InstrumentHandlerCounter(
 		httpCnt.MustCurryWith(prometheus.Labels{"handler": "static"}),
-		http.FileServer(root),
+		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			r.URL.Path = r.URL.Path[len(prefix):]
+			handler.ServeHTTP(w, r)
+		}),
 	)
 }

main.go

@@ -96,7 +96,7 @@ func main() {
 	r.POST(pushAPIPath+"/job/:job", handler.Push(ms, false))
 	r.DELETE(pushAPIPath+"/job/:job", handler.Delete(ms))
 
-	r.Handler("GET", *routePrefix+"/static/*filepath", handler.Static(asset.Assets))
+	r.Handler("GET", *routePrefix+"/static/*filepath", handler.Static(asset.Assets, *routePrefix))
 
 	statusHandler := handler.Status(ms, asset.Assets, flags)
 	r.Handler("GET", *routePrefix+"/status", statusHandler)