feat: add series label component

Adds a chart series label component. We need to use the same label in our chart tooltips and legends. Creating this in a reusable component will ensure consistent UX across all of our charts.

This should not introduce any visual changes to existing snapshots as it simply moves what we are already doing into a reusable component:

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Relevant Issues

Part of resolving https://gitlab.com/gitlab-org/gitlab-ce/issues/52258