Fix metrics when using serverless-init

[terlenbach]

What does this PR do?

With the start time of the flush set to start + 10s, we trick the time sampler, and make it believe that the current bucket is closed, making it reporting wrong metrics.

This PR fixes #19446

Motivation

Fixing the issue #19446

Additional Notes

This PR also removed a comment stating the reason for these extra 10 seconds.
It says: "We add 10 seconds to the interval to ensure that we're getting the whole sketches bucket".
My gut feeling is that this reason is no longer valid, but I couldn't find in the git log the bug that this fixed, so I couldn't dig deeper.

Possible Drawbacks / Trade-offs

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[bits-bot]

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