fix axonlog flaky test (2)
fix axonlog flaky test (2)

Summary:
Flaky test: https://www.internalfb.com/intern/testinfra/diagnostics/281475227943443.562950013806381.1657748173/

Most of these tests that rely on stats assertions are prone to flakiness.
This one especially since it asynchronously calculates rate - see https://www.internalfb.com/intern/test/562950013806381/
I don't see other way to make this more stable than adding a sleep, although understandably it's bad fix. Open to suggestions how to fix this better.

Reviewed By: disylh, stuclar

Differential Revision: D37839007

fbshipit-source-id: 3fda2099f596e46a399f5d8b1a6990a3c6480f37

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter