fix(metric): fix rendering artifact

[nickofthyme]

Summary

Fixes minor rendering artifact in Chrome introduced in #2184

Details

With the changes from #2184 we introduced support for transparent colors. This change would have overlapping colors on the trend line metric causing blended colors. To prevent this we split the top and bottom half of the sparkline in two so the shapes rendered independently directly on the metric background.

Doing this caused a slight white sliver to be rendered between the two halves for colors with hight constant to the background, even with strokeWidth set to 0.

To fix this we render a thin line along the spark line to fill this render defect without making the line too thick where a transparent color overlap would create a noticeable line.

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The code has been checked for cross-browser compatibility (Chrome, Firefox, Safari, Edge)
• Visual changes have been tested with light and dark themes

[nickofthyme]

buildkite update screenshots

[nickofthyme]

Closing this in favor of #2255