DB testing: Refactor to have stateful observers per migration

What does this MR do?

Related to gitlab-org/database-team/gitlab-com-database-testing#24 (closed)

Refactor lib/gitlab/database/migrations/instrumentation.rb so that it creates an observer per migration, passing in the details of the migration being observed.

Test comment: !60842 (comment 624370478)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• [-] This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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Security

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• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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