Support multiple queries on a single chart in Metrics

What does this MR do?

• Groups metrics based on shared attributes (y_label, group, and title) in the UI.
• Supports create/read/update/delete actions for multiple metrics per alert dropdown

CE Port

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25758

What's still missing?

• actually updating common_metrics.yml to transition canary metrics back to a single chart (Separate MR) - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25771
• adding unit/acceptance tests
• changelog
• commit cleanup
• CE port - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25758
• some more general cleanup I'll comment on in-line

Some screenshots

multiple queries	existing alert	alert dropdown	empty alert	new alert

Local Testing

Prereqs: A project running Prometheus w/ deployed envs (follow setup instructions then create an MR)

1. Checkout branch
2. Open rails c
3. Check that you have n metrics locally: Run PrometheusMetric.count
4. Add a new query to one of the existing metrics in common_metrics.yml (I just copy & paste one then multiple it by 0.85)
5. Run:

require Rails.root.join('db/importers/common_metrics_importer.rb')
Importers::CommonMetricsImporter.new.execute

1. Confirm that you now have n+1 metrics: Run PrometheusMetric.count
2. Navigate to the monitoring page for your environment (Operations > Environments > tiny chart icon || Show More > tiny chart icon)
3. You should already see an additional line on your first graph which just represents 85% of the value of the chart.
4. Cleanup: Run PrometheusMetric.last.destroy

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/54877

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [n/a] Conforms to the database guides
• [n/a] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• [n/a] 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