Fix styling for heatmap chart on metrics dashboard

Following the conversation from related issue #214312 (comment 323333020)

We can allow them to stretch full column size.

That looks a lot better, to me. We could perhaps also have them be the full width of the container by default, as well, so there could only be one heat map per row. This might also help with the issue of there not being enough room for the x-axis labels to display.

Stretch heatmap charts to full column size on metrics dashboard