fix(logging): Add back constants for backward compatibility

[zmyzheng]

Summary

Before v1.31.0, Telegraf had some public constants inside logging package: https://github.com/influxdata/telegraf/blob/release-1.30/logger/logger.go#L20~#L23. Starting from v1.31.0, these constants are removed and hard-coded strings are used instead in the code. This seems to be a breaking change for some projects referring to these constants. This change is not announced in the release announcement of v1.31.0 as well. It would be great if we can add back those constants to Telegraf source code to have a consistent behavior as the historical code and avoid hard-coded string in the source code.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #15513

[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 increases the Telegraf binary size by 1.60 % for linux amd64 (new size: 243.5 MB, nightly size 239.7 MB)

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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

[powersj]

I've approved and will have others review, but again as I mentioned we don't guarantee backwards compatibility in these parts of the code. I fear we may unintentionally break you down the road again.