[FLINK-38051] Fix flaky test JobAutoScalerImplTest

[mateczagany]

What is the purpose of the change

I often had JobAutoScalerImplTest#testMetricReporting fail on my local computer, this happens when there is less than 1ms between the two metrics point being added, and thus ScalingMetricEvaluator#getRate trying to divide with 0.

I think it makes sense round up the duration to 1 ms in this case, even though it's not really expected to happen in a real scenario.

Brief change log

• Update ScalingMetricEvaluator#getRate to round up the difference between the first and last timestamps to 1 ms.

Verifying this change

• Verified locally that the test is no longer flaky on my machine.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changes to the CustomResourceDescriptors: no
• Core observer or reconciler logic that is regularly executed: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable