Improve chart popover consistency between components

We currently have a couple different versions of popovers in charts.

Stacked Column	Area	Line

What's happening in the stacked column chart example is closest to the original design, where the color bar and value name are in a separate column to the value displayed.

Now that our popovers have gone through a few iterations since creating this issue, the way they appear on the metrics dashboard is how the defaults should appear in gitlab-ui. Primarily the issue is the way the popovers display the chart series label in relation to their values (in two, distinct columns). The chart types that need updated are:

• stacked column
• line
• area