fix: snmp input plugin errors if mibs folder doesn't exist (#10346)

[sajoupa]

Required for all PRs:

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

resolves #10355

When there is no MIB folder, the snmp input plugin with exit with an error: e00147d.
This change makes it log a warning and continue (the plugin works fine even without MIBs).
It also adds a unit test which would have caught the issue.

[powersj]

@MyaLongmire @reimda what is the behavior we want? Master currently stops Telegraf is a MIB is not found, this changes that to log the error, and keep processing.

[reimda]

@powersj We want it to be possible to use the snmp plugin without mibs, but also for telegraf to fail with an error if the user intentionally set the mib directory in telegraf.conf but made a typo or entered an invalid mib directory. It looks like this PR goes too far and makes all directory errors into warnings.

[MyaLongmire]

Is there a reason this got moved?

[sajoupa]

Yes, in case we don't find any mibs, we don't want to append mibPath to folders.

[telegraf-tiger]

👍 This pull request doesn't change the Telegraf binary size

📦 Looks like new artifacts were built from this PR.

Expand this list to get them here ! 🐯

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_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
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
		static_linux_amd64.tar.gz

[powersj]

Thank you!

[Hipska]

@reimda @powersj @MyaLongmire Could it be that since this change, snmp_traps isn't loading anymore? #10548