feat(processors.template): Unify template metric

[srebhan]

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

resolves #13034

This PR unifies the template metrics as far as possible currently. The PR deprecates the FieldList and TagList functions as those are already existing with a different signature in metric. Once those functions are removed we can remove the surrounding TemplateMetric implementation in processors.template and use the metric implementation.

[Hipska]

I think that's nicely solved, just one question..

[telegraf-tiger]

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⚠️ This pull request increases the Telegraf binary size by 3.46 % for linux amd64 (new size: 193.8 MB, nightly size 187.4 MB)

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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