WIP: Migrate/import multiple queries - Part 2

What does this MR do?

Part 2 of supporting multiple queries per chart. Part 1 blocks Part 2, which blocks Parts 3A & 3B.

As of 2/15, this MR is against the Part 1 branch setup-multiple-queries for simpler review. Once Part 1 has been merged, I'll update the target branch & try to remember to update this description.

Part 1: setup-multiple-queries

• add new prometheus_queries table

• add new class, factory, specs

Part 2: import-multiple-queries

• migrate existing query data from metrics table to queries table

• update PrometheusMetric#queries to return contents of queries table

• update importer to expect multiple queries

Part 3A: display-multiple-queries

• update common_metrics.yml to merge target charts & call updated importer (TBD)

• any FE work (TBD)

Part 3B: cleanup-multiple-queries

• "ignore" query columns in metrics table

• delete query columns in post-deployment migration

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
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer