Fetch the route count once, while holding the lock.

Not really a problem in practice as it'd be hard to get a second reload
running in between the `rt.lock.Unlock()` and the `rt.mux.RouteCount()`,
but this should be slightly more robust to future changes.

lib/router.go

@@ -238,11 +238,11 @@ func (rt *Router) reloadRoutes(db *mgo.Database, currentOptime bson.MongoTimesta
 
 	rt.lock.Lock()
 	rt.mux = newmux
+	routeCount := rt.mux.RouteCount()
 	rt.lock.Unlock()
 
-	logInfo(fmt.Sprintf("router: reloaded %d routes", rt.mux.RouteCount()))
-
-	routesCountMetric.Set(float64(rt.mux.RouteCount()))
+	logInfo(fmt.Sprintf("router: reloaded %d routes", routeCount))
+	routesCountMetric.Set(float64(routeCount))
 }
 
 func (rt *Router) getCurrentMongoInstance(db mongoDatabase) (MongoReplicaSetMember, error) {