fix(inputs.prometheus): respect selectors when scraping pods

[redbaron]

Required for all PRs

• Updated associated README.md.
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field and labels selectors were ignored unless pod_scrape_scope = "node" was specified, this breaks monitor_kubernetes_pods_method != "annotations" (default), because then all pods become candidate for scraping.

This PR enables field and label selectors. I am keeping existing behaviour where field and label selectors are not used with default pod scraping (monitor_kubernetes_pods_method == "annotations") for now, but I'd prefer any pod scraping to respect selectors same as it currently respects namespace.

[telegraf-tiger]

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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_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
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

[srebhan]

Looks good to me. Thanks for the quick fix @redbaron!