fix(parsers): Memory leak for plugins using ParserFunc.

[srebhan]

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

resolves #11809

When input plugins (or processors) use the passed ParserFunc, the new parser gets added to the list of running parsers in config.Parsers. As this instantiation of parsers can happen often and is out of the control of Telegraf's general framework, the array of running parsers will keep growing and is newer cleaned up. This means that a reference to the no-longer-used parsers is kept, garbage collection cannot free the parsers' space and we slowly eat up all memory.

The PR completely removes tracking the running parsers and thus solves the memory leak. However, once we want to go to dynamic plugin starting/stopping this problem will hit us for other categories as well as there is currently no way to remove running plugin instances from the corresponding lists.

[sspaink]

@srebhan why was Telegraf keeping track of the running parsers?

[srebhan]

@sspaink when moving to the more plugin-like structure we used it to initialize the parsers just as we do with the other plugins, but I don't think it is necessary.

[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_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