Ultrasonic sensor for metal? It's possible ?

I already know the answer is yes.

But someone knows what type of sensor can measure a liquid externally.

Like this:

https://www.youtube.com/watch?v=A3swYDQRzFk

What do you want to do? You want to sense metal or liquid inside metal?

sense liquid level inside metal

Try giving Maxbotix sensors a browse. Should help you out.