'Polls over time' graph should be a rate, not a total

[sd2k]

The polls over time graph currently just shows the latest value of the 'polls' field, which is effectively a counter, so it just increases forever. It would be much more useful to see a polls/second rate over the last 5 minutes or something similar. I'm not sure if this is possible in the Grafana transformations, so we might need to do it in the backend.