Allows average and max legend text in time series wrapper to be overwritten by props

What does this MR do?

Adds the legendText prop to the Time Series component. This is to enable internationalization of the abbreviations Avg and Max. The prop will be passed down all the way to the component in Gitlab-UI, which in turn will pass it to the Legend component.

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides

gitlab-ui#245 (closed)

MR that adds the props to the component: gitlab-ui!705 (merged)