fix(agent): Warn on multple agent configuration tables seen

[powersj]

Summary

Users sometimes try to use multiple agent sections, but this leads to undefined behavior. We should start to warn and eventually error on these types of issues.

Checklist

• No AI generated code was used in this PR

[Hipska]

Isn't that doing just a merge of the 2? I see no issue in having these configs:

[agent]
interval = "30s"

[agent]
debug = true

[powersj]

I see no issue in having these configs:

Yes, your hand-crafted, limited example would not present any issues. It however gives the appearance that you can have multiple independent agent definitions, which is simply not true.

[Hipska]

Sure, but I don't agree to "eventually error on these type of issues". I do have a valid use case to have multiple agent sections.

I agree to throw warnings or error out if those sections have overlapping parameters/items.

[telegraf-tiger]

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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