feat(outputs.mongodb): Add PLAIN authentication support and validation

[skartikey]

Summary

Added PLAIN authentication support following the same pattern as SCRAM:

  [[secretstores.os]]
    id      = "mongosecret"
    keyring = "telegraf"

  [[outputs.mongodb]]
    dsn = "mongodb://${MONGODB_HOST}/?tls=true"
    authentication = "PLAIN"
    username = "@{mongosecret:mongo_username}"
    password = "@{mongosecret:mongo_password}"

Security Considerations

PLAIN authentication sends credentials in plaintext during the SASL authentication handshake. The implementation includes:

• Runtime warning if tls=true or mongodb+srv:// is not detected
• Documentation warnings about TLS requirement
• No change to how credentials are transmitted (follows MongoDB driver behavior)

Users must use TLS to secure PLAIN authentication in transit. Secret store protects credentials at rest.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #6460

[srebhan]

Thanks @skartikey! Some comments...

[srebhan]

Awesome job @skartikey! Two small suggestions...

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_arm64.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%)

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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]

Perfect. Thanks @skartikey!