Provide rate information directly

[wilaw]

Current metrics all provide an absolute count of frames. To be useful for player developers as a switch trigger, players will usually need to convert this to a rate. It would be helpful if the interface provided this information directly. The values are obviously dependent on the timebase chosen, but something in the range of 1s would provide a faster reaction to problems and players could average this value over a longer period if they preferred .

Suggest

readonly attribute unsigned double droppedVideoFrameRate;
readonly attribute unsigned double totalVideoFrameRate;