feat: add partition_of tag to postgresql.seq_scans metric

[Chrisp1tv]

What does this PR do?

At Doctolib, we need the partition_of tag for metric postgresql.seq_scans and found out it's not available 😢

Doing a PR to have it now 🎉

I would appreciate a lot some help to implement tests, as I'm not used to the structure of project nor to Python TBH :)
Could you point me the files I should update to test the new behavior?

Motivation

Using the partition_of tag in our monitors, dashboards and so on, of course! 😄
And to make Datadog even better than it already is! 🎉

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.70%. Comparing base (fdf1f78) to head (93e0637).
Report is 202 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
postgres	93.31% <ø> (+4.01%)	⬆️

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[Chrisp1tv]

Hey 👋

As mentioned in the PR description, I would appreciate a lot some help to implement tests, at least a pointer to the files I should update to test the new behavior, and fix the broken tests.

Or if you prefer to update tests yourself, that works for me too, no problem! 😄

[azhou-datadog]

Hey @Chrisp1tv 👋 ! Thanks for contributing :)
So few things - you will need to update the changelog to pass the pipeline, by running ddev release changelog new, see Details

It looks like this change will add the partition_of tag to not just postgresql.seq_scans, but also all the other metrics in the QUERY_PG_CLASS columns section. So should include that in the description of PR and changelog.
You should also update the descriptions of the corresponding metrics that have this new tag in postgres/metadata.csv

I merged your branch with master and that seems to have fixed the broken tests. As for adding new testing to cover this new tag, take a look at postgres/tests/test_relations.py and how the QUERY_PG_CLASS query is tested, especially with the expected_tags.
You may also need to take a look at the files that set up the Postgres test instance for the unit tests at postgres/tests/compose/resources/

[Chrisp1tv]

Hey @azhou-datadog 👋

Many thanks for the clear instructions 🙏 I'll try to have a look when I have time and get back to you if I need help!

[iliakur]

@Chrisp1tv I've added the "WIP" label to this PR, we can remove it after you have time to implement changes.