Add threshold lines in metrics graph

What does this MR do?

Allows setting threshold in new echarts version of metric graphs:

CE Backport

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24169

What are the relevant issue numbers?

Closes #9054 (moved)

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer