Update chart popover (tooltip) default titles

Follow-up issue from #420 (comment 226968983)

I wanted to summarize our conversation from this morning on this issue: I agree, both the values and titles are needed in the popovers, and we should update the chart popover components in GitLab UI to reflect this.

Looking again at the original spec, I think there is an error in the design for the single column chart popovers, which I will fix asap. But, the original intention was to have the following information in popovers:

Header x-axis value + x-axis title

Body section y-axis title + y-value

It seems like this would also align with your needs for Productivity Analytics.

It looks like this issue, in particular, is just about adjusting the "body section" of the popover so that it has both a title and a value. Do we need a separate issue to ensure the "header" section also has a title and a value?