feat(inputs.sflow): Add default source tag

[Hipska]

Required for all PRs

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

Related to #8011 and #4413

Add the source tag to standardise the sflow input.

[telegraf-tiger]

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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

[srebhan]

@Hipska while I love the PR, the issue is that you double the cardinality (at least for InfluxDB) users with this change. Can you please add an option to switch between the existing tag and the new source tag!

[Hipska]

I did follow METRIC_FORMAT_CHANGES doc, it is not considered to be unsafe (whose need such an option).

[powersj]

it is not considered to be unsafe (whose need such an option).

While adding a tag is not specifically listed as "unsafe" it is listed as "be careful with
cardinality". The addition of this tag is not strictly necessary. In order to prevent issues who may already be at a limit of cardinality for whatever reason, then it should follow the unsafe change guidelines:

If your change has the potential to seriously affect existing users, the change must be opt-in.

[powersj]

@Hipska same question here -do you want to make it a doc change to suggest using "source" or close this?

[Hipska]

Well, if the idea is to deprecate this plugin, then its best to just close this PR I guess..