[cert_manager] Migrate from integrations-extras

[davidor]

What does this PR do?

Migrates the cert-manager integration from integrations-extras.

I have not changed the code. All credit goes to @arapulido who created the integration and all the other contributors.

I just adapted a few things like the install instructions in the README, the maintainer email, etc.

This is my first time migrating an integration from the integrations-extras repo, so I have some questions:

• I have set 1.0.0 as the version number and reset the CHANGELOG. Is that OK or should we continue from the version in integrations-extras?
• I've changed the author to Datadog because I've seen that's what we use in the integrations of this repo. Is that OK or should we keep the original in this case?

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

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Codecov Report

Merging #12344 (39a8098) into master (a05ca18) will decrease coverage by 0.01%. The diff coverage is 77.41%.

Flag	Coverage Δ
cert_manager	77.41% <77.41%> (?)

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[hithwen]

Can you add container integrations as codeowners for this?

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[davidor]

@sarah-witt Thanks for the review. I've addressed your comments:

• Deleted requirements.in.
• Deleted MANIFEST.in.
• Updated setup.py with some things that were missing from the original one. I wasn't sure about what's no longer needed.
• Removed the pip install -r requirements.in line from tox.ini.
• Updated the datadog-checks-base dependency requirement to >=25.1.0. I don't know which version we should use exactly, so I chose this one which is the one that most integrations are using.
• Added the spec in manifest.json.
• Regenerated the example spec. My understading is that this integration should use openmetrics_legacy because I've seen that it accepts prometheus_url as a config param.

I've added each thing in a different commit.

[hithwen]

Reference

Has this been addressed? I think we should still bump the version to generate a new release from core, but we should definetly preserve prior history

[davidor]

@hithwen I added a new commit that brings the changelog from integrations-extras and continues with the same version number.

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[davidor]

The CI is failing because there's a check that detects that there's already an integration with the same name, which is expected because we know that it exists in the integrations-extras repo.

Apart from that, I pushed 3 new commits that were added after the last approval by @FlorentClarret

• Migrate to OpenMetrics V2 (as suggested by @ofek ).
• Adapt to changes in tooling.
• Migrate manifest to v2.0.0 (it's now required to make the tests pass).

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