Chart popovers: display additional metrics

Problem

• Currently, chart popovers only displays information from the Y-value of the chart
• For some use cases, there is a need to highlight additional data that is relevant but not shown in the chart
• Example: gitlab-org/gitlab#296878

Proposal

• Allow chart popovers to optionally display additional related data underneath the Y-value (Y-value should always remain at the top)
• Limit to 5 metrics (Y-value + 4 additional) maximum
• Also allow for an optional description (and documentation link) to be added below the metrics
• Update PJs documentation