Fix AdaptiveAllocationsScalerServiceTests.test_scaleDownToZero_whenNo…

[jan-elastic]

fixes #136907

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[jan-elastic]

These two settings:

• timeIntervalSeconds of ONE_SECOND
• scaleToZeroAfterNoRequestsSeconds of ATOMIC_SECOND

lead to a race condition. Adaptive allocations service checks after 1 second (the timeIntervalSeconds) whether there's been no request for 1 second (the scaleToZeroAfterNoRequestsSeconds).

On Unix-based systems this seems to always pass. Locally on my MacBook the timeWithoutRequestsSeconds is always something like 1.001 seconds, so the model is scaled to zero.

This test is probably failing on Windows builds on x86 hardware, because of a less precise system clock, see:
https://learn.microsoft.com/en-us/windows-hardware/drivers/kernel/high-resolution-timers#controlling-timer-accuracy. Not sure 100% sure, I'm no Windows expert by any means.

Increasing the timeIntervalSeconds to 2 seconds, and then checking whether there's been no request for 1 second, solves the issue. Unfortunately, this add 2 more seconds of unit test time, because the times are set in seconds instead of milliseconds.

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
✅	9.2
❌	8.19	Commit could not be cherrypicked due to conflicts
✅	9.1

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 137731