Add kube_job tag to job metrics

[ahmed-mez]

What does this PR do?

Adds kube_job tag, it takes the value of job_name.

Motivation

Match what the tag key used by the tagger

Additional Notes

fixes: DataDog/helm-charts#327

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

[codecov]

Codecov Report

Merging #9775 (202f8f1) into master (2085730) will increase coverage by 0.00%.
The diff coverage is n/a.

Flag	Coverage Δ
datadog_checks_dev	79.93% <ø> (ø)
kubernetes_state	89.69% <ø> (ø)
snmp	91.57% <ø> (+0.04%)	⬆️

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