fix(inputs.systemd_units): Revert to only gather loaded units by default

[srebhan]

Summary

With PR #14814 we also produce metrics for unloaded units. However, for unspecific filters (like * being the default) this touches a lot of files and forces systemd to lookup those unit-files which is expensive.

Therefore, this PR reverts the behavior to pre-v1.30.0 versions and only collects units already loaded by systemd. To allow collecting disabled/unloaded units you can use the new collect_disabled_units.

Additionally to this change, the PR also optimizes the interaction with systemd for cases where non-detailed metrics are requested.

This leads to the following improvements when gathering all units (pattern = "*") in non-detailed mode

goos: linux
goarch: amd64
pkg: github.com/influxdata/telegraf/plugins/inputs/systemd_units
cpu: AMD Ryzen 7 3700X 8-Core Processor 

version	performance	improvement
v1.30.0 reference	2,021,195,198 ns/op	1.0x
all units including non-loaded	700,185,759 ns/op	2.9x
only loaded units (default)	5,754,030 ns/op	351.3x

Checklist

• No AI generated code was used in this PR

Related issues

resolves #15104
resolves #15093

[powersj]

Thanks for jumping on this. As a PR this looks fine, but I'm still concerned that the usage is still higher than it used to be. Do we need to have this new method be opt-in?

[srebhan]

@1tft FYI: This PR disables collection of non-loaded/disabled units by default, so in case you need them please use the new collect_disabled_units option! Would be cool if you can test this in your environment...

[powersj]

Thank you - one question about a println statement?

[1tft]

@srebhan Thank you so much for keeping that feature alive. With this build and collect_disabled_units = true we can collect metrics for unloaded units.
Only one remark for Readme: Maybe you should also mention that collect_disabled_units = true activates metrics for systemd_units with state=disabled AND with state=static.

[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 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]

Thank you!