feat(outputs.mqtt): add support for MQTT 5 publish properties

[jlordiales]

resolves: #12526

Add optional support for MQTT 5 publish properties in the config

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

[serroba]

FYI @svagner
🙇

[powersj]

Thanks for taking the time to put this together! I have a couple of changes and comments in-line.

[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 significantly change the Telegraf binary size (less than 1%)

📦 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

[powersj]

Thank for the quick turn around!

@serroba you were the one who filed the feature request, could you give the artifacts in this PR a try and confirm they resolve your request?

[jlordiales]

I've verified this is working as expected by having telegraf write to a local mosquitto instance.

With:

mosquitto_sub -F "Topic content type: %C,\n Message expiry: %E,\n User properties %P,\n Response topic: %R,\n %J" --protocol-version 5 -h mosquitto -p 1883 -t "<topic>"

and the new config properties:

[outputs.mqtt.v5]
  content_type = "application/json"
  response_topic = "test-response-topic"
  message_expiry = "10s"

[outputs.mqtt.v5.user_properties]
  "key1" = "value 1"
  "key2" = "value 2"

we get the expected values from the server:

Topic content type: application/json,
 Message expiry: 10,
 User properties key1:value 1 key2:value 2,
 Response topic: test-response-topic,
<payload>

@serroba and I work together :)

[serroba]

Thanks for checking with me @powersj
Yes, indeed, we work together.

Changes LGTM. Thanks a lot to both!

[srebhan]

Looks good to me. Thanks for the contribution @jlordiales!