fix(inputs.gnmi): Add option to explicitly trim field-names

[srebhan]

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

resolves #13512

This PR adds an option to explicitly trim leading dots and slashes from field-names defaulting the new option to false (i.e. keep the leading dots and slashes). This is required to fix a regression where the field-names contain a leading slash for Telegraf versions before v1.27.0 in case a subpath matched the subscription. In this case the field-name still contained a leading slash.

[srebhan]

@protonmarco please note that you set the new trim_field_names option to true in your use-case. Sorry for the inconvenience, but we need to do it to fix a regression for all other users...

[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

[protonmarco]

@srebhan Thanks for the heads up

[powersj]

Will let you merge, but wanted to let you know I'm +1 before long weekend

[clsmyth]

Fix verified on my end; i am reporter of #13512