fix(agent): keep collection interval fixed even when jitter is applied.

[redbaron]

Summary

This changes behaviour of collection_jitter to be more like collection_offset, but with the offset calculated randomly for each plugin.

Net effect is that each plugin collection interval will be fixed at interval value, but collection times across all plugins will be spread over collection_jitter range.

Checklist

• No AI generated code was used in this PR
• AI generated code used in this PR follows the InfluxData Policy on AI-Generated Code Contributions

Related issues

resolves #17287

[telegraf-tiger]

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i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
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ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
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[skartikey]

@redbaron Thanks for this PR! The change looks good and addresses the issue well. One thought:
The existing tests will still pass, but don't explicitly verify the new behaviour. It might be worth adding a test or two to capture the changed semantics - something that verifies the interval between ticks remains fixed even with jitter configured, or that the jitter offset stays consistent across multiple ticks. Just a suggestion - feel free to approach it however makes sense to you. The existing mock clock infrastructure in tick_test.go should make this straightforward.

[skartikey]

@redbaron Could you please rebase this PR onto the latest main branch? There is a failing test on the current base, and rebasing should pick up the fix.