fix: race condition in WindowedThroughput sampler

[kentquirk]

Which problem is this PR solving?

• A "flaky" test in Refinery would fail occasionally with a race condition in WindowedSampler. This fixes it.

Short description of the changes

• Use a local temporary variable instead of modifying the count on the fly, then write it during the existing lock periods.

[fchikwekwe]

Oh, I see. Was this an issue with adjusting the struct values in place?

[kentquirk]

Basically, yes. It was changing t.numKeys without a lock around it. I could have locked it there, but instead I used a temp and modified t.numKeys inside the already-locked regions so that we didn't have to acquire another lock.