Adds new pod metric kube_pod_container_state_started

[vikkyomkar]

What this PR does / why we need it:
This adds a new metric kube_pod_container_state_started under POD metrics

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

Fixes #1167

[brancz]

Could you increase the number of expected metrics in the benchmark test? We model the benchmark around the Pod collector and test for correctness as well. You just need to increase the 56 metrics to 57.

[vikkyomkar]

Could you increase the number of expected metrics in the benchmark test? We model the benchmark around the Pod collector and test for correctness as well. You just need to increase the 56 metrics to 57.

Thanks for the hint @brancz
Now this PR is ready for review

[brancz]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brancz, vikkyomkar

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [brancz]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment