feat(parsers.avro): Allow union fields to be specified as tags

[fajpunk]

Summary

Flatten any fields specified in avro_tags before trying to convert their values to strings.

Without flattening, any union fields included in avro_tags do not actually get converted into Influx tags. This warning is logged:
2024-12-06T19:43:12Z W! [parsers.avro::file] Could not convert map[string:some_value] to string for tag "some_union_in_a_tag": type "map[string]interface {}" unsupported

Checklist

• [x ] No AI generated code was used in this PR

Related issues

resolves #16271

[telegraf-tiger]

Thanks so much for the pull request!
🤝 ✒️ Just a reminder that the CLA has not yet been signed, and we'll need it before merging. Please sign the CLA when you get a chance, then post a comment here saying !signed-cla

[fajpunk]

!signed-cla

[athornton]

This looks OK to me. I don't know that I can actually review it since I don't have write access to influxdata:master, and we definitely need some InfluxData eyes on it, but I (or, I presume Dan) would be happy to talk anyone from InfluxData through what we're doing and why.

I think it's funny that it's our own team that eventually demanded an answer to Angelo's question

I have a question that comes from @afausti -- what should the default behavior be if you explicitly mark an incoming field as both a tag and a field? The current code "does both" which ... I don't know what that does in practice. The right answer is "don't do that," of course...but should I catch it when building the parser and warn and leave it a tag but not add it as a field?

from #13945

[athornton]

Well, OK, not quite the same question, it's "what do we do if a field is not a simple type", but turning it into a string somehow is clearly the right answer and this seems like a pretty good way.

[telegraf-tiger]

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Downloads for additional architectures and packages are available below.

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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

[srebhan]

Awesome contribution @fajpunk! Thanks you!

@athornton thanks for the review!

I relabeled this as a feature as this is what it is in my view. @DStrand1 what do you think?