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 87.10%. Comparing base (aa9bd4e) to head (33c2a61).

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
hive	?
hivemq	?
ignite	?
jboss_wildfly	?
kafka	?
postgres	77.58% <ø> (-11.72%)	⬇️
presto	?
solr	?

Flags with carried forward coverage won't be shown. Click here to find out more.

[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! 😄