implement session stats, fixes last (admin|user) left at won't work for any provider that removed users after logout.

server/internal/api/session.go

@@ -103,3 +103,8 @@ func (api *ApiManagerCtx) UpdateProfile(w http.ResponseWriter, r *http.Request)
 
 	return utils.HttpSuccess(w, true)
 }
+
+func (api *ApiManagerCtx) Stats(w http.ResponseWriter, r *http.Request) error {
+	stats := api.sessions.Stats()
+	return utils.HttpSuccess(w, stats)
+}