feat(migrations): Add migration for inputs.snmp_legacy

[srebhan]

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

relates to #13376

This PR adds migrations for the deprecated inputs.snmp_legacy plugin that requires manual intervention.

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

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

📦 Click here to get additional PR build artifacts

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

[Hipska]

Am I right that in these cases it is opted for telegraf to just silently (Inform message in log) ignore the plugin rather than preventing telegraf to start up?

[srebhan]

I don't think we should silently ignore these but instead fail early and loudly. The user can still comment the plugins or filter the inputs via command-line option.

[Hipska]

I don’t see that actually. Could you point where this fail is happening?

[srebhan]

The fail happens as we do not remove the plugin and telegraf will (or at least should) fail on start when we rollout v1.30.0 on the deprecated plugin.

[Hipska]

Okay, I think I'm starting to get it.. But not completely yet.. 😂

So, why is this migration created if it's not possible to migrate? And did I also understand correctly that the plugin code will not get actually removed in 1.30?

[srebhan]

So, why is this migration created if it's not possible to migrate?

I created the migration in case someone runs the migrate command and expects that afterwards the config works. Those people will now (hopefully) see the message and fix the issue themselves. Otherwise they might expect that Telegraf will just work...

And did I also understand correctly that the plugin code will not get actually removed in 1.30?

We will discuss this. One idea is to keep the code in 1.30.0 (and get an error message) and remove it in 1.30.1 for the case that some unforeseen issues appear. The (more likely) alternative is to remove the code directly in revertible PRs.

[Hipska]

Many thanks for the explanations.. I now fully understand.