Add replicaset label metrics

[kminehart]

What this PR does / why we need it: This PR adds the kube_replicaset_labels metric. In some edge case scenarios this can be useful.

In our environment, we heavily use labels for tenancy. Developers want to be able to see how often they "deploy" an update; this can normally be done using kube_deployment_created, unless the deployment is updated. When the deployment is updated, a new replicaset is created, so we use kube_replicaset_created, but we can not determine what team owns the replicaset without being able to access its labels

I'm having minikube issues so I'm unable to run the e2e tests locally 😅

[kminehart]

I'm unsure why the travis build failed, it does not seem relevant to the changes I've made.

/assign @mxinden

[mxinden]

Thanks for the contribution @kminehart.

This looks good to me. Adding this single metric should be fine, given that we already have the replicaset dimension in our cardinality set.

Still giving @brancz @andyxning @zouyee a chance to reply before merging.

The test failure was a flake.

[brancz]

I'm having minikube issues so I'm unable to run the e2e tests locally 😅

Please let us know how we can improve docs to make this easier for the next contributor :)

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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