Most photographers rely on the in-camera meter. Some use a light meter for studio or formal shots like wedding altar. With video if you're constantly changing scenes and location I would think the camera's metering would be easier.
The following uses Nikon terms, I don't know Canon's or others terminology. In changing light try using Aperture Priority. The Aperture (f/stop) will remain constant while the camera adjusts shutter speed. Shooting video it may also automatically adjust ISO depending on menu settings. While in Aperture Priority you can make adjustments to changing light by adjusting Exposure Compensation. EC changes the exposure in 1/3 stop increments. I like shooting in fully manual 90% of the time but in quickly changing light, Aperture Priority is the way to go.
The tools and techniques may change, but the principles of light and exposure remain the same. In contrasty situations, if you have the opportunity, a good meter, such as those available from Sekonic, can be used to take an incident light reading (reading the light falling on the subject instead of what's reflected off it) will give you a more accurate reading. In a pinch, and much more economically, go to a good camera store and buy an 18% gray card. Put it in the picture and take a reflected reading off of it, the effect is the same as an incident reading. Buy a couple of cards as they are usually cardboard and will wear out. You can also buy two-sided cards, white and gray, to take exposure and set white balance.
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