Demo Site: show plausible live demo metrics

[rib]

Currently the demo site doesn't show how the bar graphs, and trace graphs work which limits how representative the demo is of what can be seen on real hardware.

Similar to how we support generating fake data when running gputop --fake it would be good if the demo site were fed a stream of fake metrics to process.

Ideally I think we'd fake the data at same level as we do for --fake by generating fake i915 perf records including OA reports and we'd hook into the code that would normally receive the web socket messages containing these reports to instead feed it fake messages.