fix: Graylog plugin standard field serialization

[alespour]

Required for all PRs:

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

Resolves #10024.

Subtle fix of fields serialization to make it compliant with GELF spec.

[telegraf-tiger]

😄 This pull request doesn't change the Telegraf binary size

📦 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

[srebhan]

Solved with #10209.