Import common metrics into database.

What does this MR do?

Migrates all built-in metrics to the database to provide a common interface for dealing with all Prometheus Metrics.

This is backport of that functionality into CE which is required by EE to implement: https://gitlab.com/gitlab-org/gitlab-ee/issues/6948.

This MR adds two extra columns:

1. common: true|false: indicates if this is system-wide or project-wide metric,
2. identifier: string: unique string to be used to identify common metric.

It imports data during system installation, and as well on-demand by preparing a database migration.

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/6948

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides