Dashboards: Look into increasing the data sampling for graphs

Using a higher data sampling allows the graphs to show more detail, but slows down the graph rendering.

With the recent improvements to graph rendering performance, maybe we can increase the data sampling a little now, but we should experiment with different rates.

The Observe > Metrics page tries to be more intelligent my using a higher sampling rate by default, but lowering it if there are many data series. Maybe we should use the same approach on the Dashboards page too.