[test/performance] round computed rate in real-traffic-test

[maschmid]

real-traffic-test benchmark SLA 2 fails frequently just because the computed rate is not exactly the expected value , e.g. https://prow.knative.dev/view/gs/knative-prow/logs/performance-tests_serving_main_periodic/1862287597506662400

SLA 2 failed. vegeta rate is 2000.006921, expected Rate is 2000.000000

Let's do the same here as we already to in https://github.com/knative/serving/blob/main/test/performance/benchmarks/rollout-probe/main.go#L234 , and add tolerance to the rate comparison to fit within the nearest integer.

• round computed Rate on real-traffic-test benchmark

[knative-prow]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.53%. Comparing base (5717d19) to head (8eed01e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15641   +/-   ##
=======================================
  Coverage   83.53%   83.53%           
=======================================
  Files         219      219           
  Lines       17427    17427           
=======================================
  Hits        14558    14558           
+ Misses       2498     2496    -2     
- Partials      371      373    +2     

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