feat(inputs.kubernetes): allow fetching kublet metrics remotely

[redbaron]

Required for all PRs

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

There is no reason to run daemonset to access kubelets, it can be done remotely. This PR makes inputs.kubernetes plugin to fetch node information from all the nodes when URL is set to empty value. Example config:

[[inputs.kubernetes]]
url = ""
bearer_token = "/var/run/secrets/kubernetes.io/serviceaccount/token"
insecure_skip_verify = true

Tested with following kubernetes cluster role:

  - verbs:
      - get
    apiGroups:
      - ''
    resources:
      - nodes
      - nodes/stats
      - nodes/proxy
      - pods
  - verbs:
      - get
    nonResourceURLs:
      - /stats/summary

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

🥳 This pull request decreases the Telegraf binary size by -2.36 % for linux amd64 (new size: 154.4 MB, nightly size 158.1 MB)

📦 Click here to get additional PR build artifacts

Artifact URLs

DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz

[powersj]

Thanks for the PR! I've added some questions due to my lack of knowledge that I am hoping you can help educate me.

Thanks again!

[powersj]

Thanks for walking me through those changes

[srebhan]

Looks good to me. Thanks @redbaron for your contribution!