Increase sample size on rate sampler test

[marcotc]

We have this flaky test in our suite:

  1) Failure:
Datadog::RateByServiceSamplerTest#test_sampling [test/rate_by_service_sampler_test.rb:35]:
Expected |132 - 100.0| (32.0) to be <= 30.0.

This failure happens because our sampling size is too small for the assertions we are trying to make.

This PR changes the sampling sample size from 1.000 to 10.000 runs. I also reduced the allowed deviation as it should go down with the increased sample size.

[delner]

👍