fix(inputs.gnmi): Handle both new-style tag_subscription and old-style tag_only

[srebhan]

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

resolves #11778

In PR #11019 a new style of handling dynamic tags was introduced, able to handle more complex tagging rules (e.g. by matching an interface key). However, the conversion/back-porting of the previous behavior will only work for a small subset of devices that expose an interface key and thus broke previous users (as described in #11778).

This PR reworks the tag-handling to be able to support both behaviors adding an match option. The plugin intelligently chooses the value if not given to be backward-compatible to both current use-cases.
However, this logic required a larger refactoring of the plugin. While code is more understandable now IMO, the volume of the change is considerable...

[cjonesdialpad]

Sent feedback via Slack thread. Summary: old tag_only syntax still not working. New syntax does get the interface description as its own item but does not seem to get applied to the actual interface data lines.

[clsmyth]

i downloaded the binary with the 3 additional commits from 18 jan 12:15...still broken for me...i will update the ticket.

[powersj]

oof this took a while, but I only have a question about all the commented out new code?

[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 doesn't significantly change the Telegraf binary size (less than 1%)

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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

[powersj]

Leaving it to you to merge, thanks for the fix!