I know this is a 4 year old answer, but I don't see anyone mention that you just have to select the toggle rulers option. Go to the settings menu option in the developer tools and check it. That will bring up the ruler icon and just click it and you have your ruler.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…