Add uid info in kube_pod_info metric

[WakeupTsai]

What this PR does / why we need it:
This PR will add pod uid in the kube_pod_info metrics.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #507

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[brancz]

You or your organization need to sign the CLA.

Beyond that, as I mentioned in the original issue, we are currently in the process of releasing a new version of kube-state-metrics, so we are currently not merging feature work.

/hold

[WakeupTsai]

@brancz It's ok. I can wait.

[zouyee]

The unit-test file need to be updated at the same time. 😄

[andyxning]

@WakeupTsai You need to also update the pod test for kube_pod_info metric to make the Travis CI green.

[andyxning]

Ping @WakeupTsai :)

[brancz]

Once CI is fixed we can merge this.

/hold cancel

[WakeupTsai]

@brancz sorry for late, I have been updated the test :)

[andyxning]

/lgtm
/approve
/hold cancel

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andyxning, WakeupTsai

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