testing: flake jobs API histogram_sample_test.py::test_histogram_sample failed

[tmatsuo]

The log:
https://source.cloud.google.com/results/invocations/7f6906ce-bd11-4d90-86c4-df9d0aaadb2c/log

[tmatsuo]

I could not repro with local 100 runs. Maybe we can ignore until we see it's happening again on Kokoro.