Use Kubernete's own func that creates config by apiserver or kubeconfig

[metalmatze]

Instead of implementing this logic ourselves we can simply leverage Kubernet's built-in function, which does exactly what we need.

We want to run kube-state-metrics outside of a cluster and simply give it an apiserver url. Right now this also needs a kubeconfig.

---

This change is 

[k8s-ci-robot]

Thanks for your pull request. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please follow instructions at https://git.k8s.io/community/CLA.md#the-contributor-license-agreement to sign the CLA.

It may take a couple minutes for the CLA signature to be fully registered; after that, please reply here with a new comment and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please sign in with your organization's credentials at https://identity.linuxfoundation.org/projects/cncf to be authorized.
• If you have done the above and are still having issues with the CLA being reported as unsigned, please email the CNCF helpdesk: helpdesk@rt.linuxfoundation.org

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[metalmatze]

Oh. I didn't see that I need to comment. 😁
CLA is signed.

[brancz]

This seems fine to me, I slightly worry about backward compatibility if we remove the --in-cluster flag, but on the other hand I expect this to be a very small amount of users and it's a change that clearly breaks and not in a weird behavior at runtime. I think I'm ok with it, wdyt @andyxning ?

[andyxning]

These two lines are unnecessary as BuildConfigFromFlags will check and log this.

[andyxning]

@metalmatze I have added a comment. PTAL.

BTW, as stated by @brancz , This is not a backward compatible change. Especially when in-cluster and kubeconfig or apiserver are both set or default kubeconfig file exists. Because after the change, kube-state-metrics will use kubeconfig config instead of in-cluster config for BuildConfigFromFlags will first use kubeconfig instead of in-cluster.

However, i also think that this should not be a big problem as we have already set in-cluster to true as the default behavior and do not specify in-cluster command line argument in kube-state-metrics deployment. Setting both in-cluster to true and kubeconfig or apiserver or there is an existing default kubeconfig file should be rarely. Overall this lgtm.

We should add a statement in the next release note to make this change widly known.

[metalmatze]

I have remove the 3 unnecessary lines.

[metalmatze]

/assign @andyxning

[andyxning]

/lgtm

/cc @brancz

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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 [andyxning]

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