Add swap memory checks to cadvisor kubelet checks

[adammw]

What does this PR do?

Add swap memory checks to cadvisor kubelet checks

Motivation

cadvisor supports swap memory metrics via prometheus that is not exposed with the datadog integration by default.

Additional Notes

Wasn't able to work out how to test our Docker agent with a separate version of integrations-core...

Review checklist (to be filled by reviewers)

• 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
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[adammw]

cc @grosser

[CharlyF]

Thanks for this contribution!

At a glance that sounds like a helpful change!

Any way you can add the metrics to this file https://github.com/DataDog/integrations-core/blob/master/kubelet/metadata.csv ?
Also, I think the new metrics have to be referenced in the test https://github.com/DataDog/integrations-core/blob/master/kubelet/tests/test_kubelet.py#L37 for the CI to be OK.

[adammw]

@CharlyF Added metrics to the metadata.csv and added tests for new metrics.