fix(inputs.chrony): Prevent race condition in concurrent gather calls

[skartikey]

Summary

The chrony plugin was experiencing intermittent panics with error runtime error: index out of range [256] with length 256 when multiple Gather() calls executed concurrently.

Root Cause:
The plugin creates a single fbchrony.Client instance shared across all Gather() calls. When Telegraf triggers a new Gather() before the previous one completes (e.g., when collecting sourcestats with multiple sources), both goroutines
access the shared client concurrently:

1. Both goroutines call client.Communicate() simultaneously
2. Both modify n.Sequence++ without synchronization (NOT THREAD-SAFE - Race condition here!)
3. Both call binary.Write() concurrently (Concurrent writes corrupt memory)
4. Memory corruption occurs during packet encoding
5. Panic occurs with index out of range [256]

This race condition is intermittent because it only manifests when gather operations overlap, typically when sourcestats collection takes longer than the configured interval.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #17757

[carsonsaldanha]

Hi, @skartikey. I have nearly an identical change as this but for the CSV parser in #17573 that's still not been addressed in over a month. A review would be much appreciated. Thanks.

[srebhan]

Thanks @skartikey! Just one small suggestion...

[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 doesn't significantly change the Telegraf binary size (less than 1%)

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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

[srebhan]

Thanks @skartikey!