Guys, I made a simple Go daemon to run in openwrt and it will create two entities in HA sensor.wan_tx and sensor.wan_rx.
For sure it open source and you can compile by yourself to your openwrt arch or if your like a mine is mips you can use the binary pre-compiled.
You can adjust the update interval if you want, the default is 2 seconds.
You just need create a Token in HA and place it in /etc/config/ha-token or use it as command line argument.