fix(parsers.xpath): Handle explicitly defined fields correctly

[srebhan]

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

resolves #13531

Currently, the fields_bytes_as_hex option only takes effect on batch definitions of fields but is ignored on explicitly defined fields. The proposed PR changes this and allows to also represent explicitly defined fields in hex-encoding.

[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_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]

@srebhan assigning to you to await user feedback

[tulku]

I tested these changes. I was able to configure:

(...)
fields_bytes_as_hex = ["sensor_samples"]
[inputs.http_listener_v2.xpath.fields]
  sensor_samples = "//n9390"

And verified that the generated metric has the sensor_samples field as an hex_string.