Fix missing metrics for static pods

[vboulineau]

What does this PR do?

A regression was introduced in agent 6.15, preventing metric retrieval for static PODs with K8S < 1.15

Motivation

Additional Notes

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

❗ No coverage uploaded for pull request base (master@6808e92). Click here to learn what that means.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
kubelet/datadog_checks/kubelet/prometheus.py	83.60% <100.00%> (ø)
kubelet/tests/test_kubelet.py	97.44% <100.00%> (ø)
kubelet/datadog_checks/kubelet/summary.py	77.77% <0.00%> (ø)
kubelet/datadog_checks/kubelet/kubelet.py	83.50% <0.00%> (ø)
kubelet/datadog_checks/kubelet/common.py	89.07% <0.00%> (ø)
kubelet/tests/test_prometheus.py	100.00% <0.00%> (ø)
kubelet/datadog_checks/kubelet/__init__.py	100.00% <0.00%> (ø)
kubelet/datadog_checks/kubelet/cadvisor.py	77.27% <0.00%> (ø)
... and 3 more