Add support for sending database_instance metadata

[alexandre-normand]

What does this PR do?

This adds sending of a new metadata kind: database_instance. This doesn't require any new queries to collect the data as it's inferred from configuration and things collected during initialization (like version).

Motivation

We'd like to keep a better inventory of database instances regardless of whether dbm is enabled or not. We already have a concept of database_instance but this makes it a more direct element of collected information rather than something that is inferred / implicit from other data we collect.

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• If the PR doesn't need to be tested during QA, please add a qa/skip-qa label.

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Test Results

     16 files       16 suites   44m 47s ⏱️
   138 tests    138 ✔️ 0 💤 0 ❌
1 104 runs  1 095 ✔️ 9 💤 0 ❌

Results for commit ac49e9c.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

Merging #15524 (ac49e9c) into master (5b46a17) will increase coverage by 0.00%.
The diff coverage is 95.23%.

Flag	Coverage Δ
mysql	87.37% <95.23%> (+0.09%)	⬆️

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