fix(inputs.gnmi): Handle canonical field-name correctly

[srebhan]

Summary

PR #14553 fixed path handling for field names. In this course the canonical-field-name handling was overlooked because now the path is no longer a simple string leading to

2024-03-07T12:44:38Z E! [inputs.gnmi] Invalid empty path "/system/memory/state/reserved" with alias "/system/memory/state"
2024-03-07T12:44:38Z E! [inputs.gnmi] Invalid empty path "/system/memory/state/used" with alias "/system/memory/state"

errors (see #14946). This PR fixes the issue and restores the previous functionality.

Checklist

• No AI generated code was used in this PR

Related issues

related to #14946

[telegraf-tiger]

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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