feat: inputs/powerdns: Add underscore option

[isodude]

When telegraf's metrics are eventually polled via PromQL,
dash in metric names are not supported.

Allow dash to be converted to underscore.

Required for all PRs:

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

In my case I'm using telegraf to send Influx to VictoriaMetrics,
which in it's turn offer PromQL to Grafana. It does not like dash in Measurements.

Current workaround is to use {name="powerdns_field-name"}, which is a hassle.
This makes it possible to use powerdns_field_name{} instead.

[telegraf-tiger]

🥳 This pull request decreases the Telegraf binary size by -1.10 % for linux amd64 (new size: 130.7 MB, nightly size 132.2 MB)

📦 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	freebsd_amd64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_armv7.tar.gz
armhf.deb	i386.rpm	freebsd_i386.tar.gz
i386.deb	ppc64le.rpm	linux_amd64.tar.gz
mips.deb	s390x.rpm	linux_arm64.tar.gz
mipsel.deb	x86_64.rpm	linux_armel.tar.gz
ppc64el.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz

[Hipska]

It doesn't seem right to adapt an input plugin to the limitations of an output plugin (or the systems behind it).

Next to that, have a look at the latest artefacts from #9561 as I think this can cover your problem as well.

[isodude]

I just found it odd that it produced the metrics with dash in them. It felt off?
I can circumvent but shouldn't fields generally be in the form field_key?

[Hipska]

That's indeed more common, but there are also plugins that produce CamelCase names..

[isodude]

What are your views around field names, should they always be the same? Or should all output plugins have to possibility to support CamelCase, dash and underscore via a generic function? Controlling output_format=camelcase|dash|underscore instead.

[Hipska]

If a specific output doesn't accept a field/tag/metric format, it should warn/error the user or convert if possible. There are processor plugins that can rename field/tag/metric names and/or values. In my opinion, there is nothing to be fixed on the input, you might want to test the given PR if that solves your issues. You could however provide a fix for the output plugin if really wanted, but I don't think this is relevant in this case as well.

[isodude]

Ok, thanks for the clear answers!