chore: Fix flaky metrics tests (#1865)

[mutianf]

Cherry pick the flaky test fix so release tests can pass.

This fixes a few flaky unit tests that relied on Thread.sleep to ensure that all metrics processing was done. Rather than using Thread.sleep, we can instead use an inline event queue in the OpenCensus stats component to execute all work inline, removing the need to wait for anything to finish.

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Fixes #<issue_number_goes_here> ☕️

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