fix(inputs.sysstat): Prevent default sadc_interval from increasing on reload

[nick-kentik]

Summary

On reload, telegraf throws away the sysstat.Sysstat instance it has and creates a new one; however, this plugin also relies on package-global state (the firstTimestamp variable), which leads to sysstat running sadc with increasingly long intervals following a reload, if the interval to use is not explicitly set via sadc_interval. Fix this by moving the firstTimestamp state into the plugin struct.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #15102

[nick-kentik]

I thought about adding a test for this - sysstat_interval_test.go is pretty close, it would just need extending with an s2 and a second sleep - but felt bad about doubling its runtime from (say) 3 to 6 seconds, so left it out.

[telegraf-tiger]

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Downloads for additional architectures and packages are available below.

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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]

Thank you for the PR!