Add graph value to hover

When hovering over a line in the monitoring graphs, I expect the values to show up in the hover box. Instead they show up in the legend, which is completely unexpected and easily missed. These values should show on hover instead.

Design

• Design should match our popover paradigm https://docs.gitlab.com/ee/development/ux_guide/components.html#popovers
• Spec files: https://gitlab-org.gitlab.io/gitlab-design/hosted/taurie/ce%2338030-spec-previews

Popover with values:

Popover with values, with bolded line when user hovers over a specific line:

Popover with values and deployment info when user hovers over a deploy: