feat(agent): Add metric disk buffer

[DStrand1]

Summary

Implements the write-through buffer persistence strategy detailed in the spec added in #14928.

Checklist

• No AI generated code was used in this PR

Related issues

Related to #802, #14805
Continuation of #15545
Supersedes #15221

[powersj]

Looking great, some initial comments reading through this.

[srebhan]

Thanks for the update and nice work @DStrand1! I have two questions...

[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 decreases the Telegraf binary size by -3.11 % for linux amd64 (new size: 243.9 MB, nightly size 251.7 MB)

📦 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

[srebhan]

Thanks for the epic work @DStrand1!

[powersj]

Thank you for the continued effort on this! There are areas I think we should continue to enhance, but we can do that in follow-ups before the release.