feat(inputs.systemd_units): Support user scoped units (#12053)

[pegaz]

Summary

Systemctl allows to monitor both system- and user-scoped units while systemd_units plugin currently supports only system units. PR aims to solve this limitation.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #12053

[srebhan]

Thanks for the contribution @pegaz! I do have some comments in the code. Furthermore, the new option needs to be added to the sample.conf file! Also do not forget to run make docs...

[powersj]

@pegaz looks like you did push some updates, other than the open README question is this ready for another review?

[srebhan]

Thanks for the update @pegaz! I have some more minor comments/suggestions...

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_arm64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

🥳 This pull request decreases the Telegraf binary size by -1.66 % for linux amd64 (new size: 239.6 MB, nightly size 243.7 MB)

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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