chore(agent): Fix buffer directory config and document

[mstrandboge]

Summary

Fixes buffer directory agent level config and documents the option.

Checklist

• [X] No AI generated code was used in this PR

Related issues

resolves #15656

[josephbrosef]

hey, i tested the new build and it doesn't seem to resolve the issue. setting buffer_strategy = "disk" and buffer_directory = "/buffer_storage" at the [agent] level does not actually change the buffer strategy. It still uses memory buffer. pretty sure config.BufferStrategy and config.BufferDirectory are always an empty string, so it will always return with a 'memory' buffer.

https://github.com/influxdata/telegraf/blob/26df1e720548ffe6662e3bfe595efd28ba5222c7/models/running_output.go#L102

https://github.com/influxdata/telegraf/blob/26df1e720548ffe6662e3bfe595efd28ba5222c7/models/buffer.go#L52-L64

Those 2 values appear to come from the OutputConfig struct (which is why i thought maybe it could be set at the Output level in the referenced bug), but i can not see how those 2 struct values are supposed to get populated with the buffer_strategy and buffer_directory from the [agent] config?

https://github.com/influxdata/telegraf/blob/26df1e720548ffe6662e3bfe595efd28ba5222c7/models/running_output.go#L25-L43

[telegraf-tiger[bot]]

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.

:partying_face: This pull request decreases the Telegraf binary size by -3.96 % for linux amd64 (new size: 252.1 MB, nightly size 262.5 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