some percentiles are not returned exactly

Special percentiles are requested here: https://github.com/envoyproxy/nighthawk/blob/main/source/common/statistic_impl.cc

It works fine at the lower percentiles, but 0.990625 seems to always appear instead of 0.99.

These are the percentiles from a couple of recent runs:

```
0
0.1
0.2
0.3
0.4
0.5
0.55
0.6
0.65
0.7
0.75
0.775
0.8
0.825
0.85
0.875
0.8875
0.9
0.9125
0.925
0.9375
0.94375
0.95
0.95625
0.9625
0.96875
0.971875
0.975
0.978125
0.98125
0.984375
0.985938
0.9875
0.989062
0.990625
0.992188
0.992969
0.99375
0.994531
0.995313
0.996094
0.996484
0.996875
0.997266
0.997656
0.998047
0.998242
0.998437
0.998633
0.998828
0.999023
1
```

Higher RPS:
```
0
0.1
0.2
0.3
0.4
0.5
0.55
0.6
0.65
0.7
0.75
0.775
0.8
0.825
0.85
0.875
0.8875
0.9
0.9125
0.925
0.9375
0.94375
0.95
0.95625
0.9625
0.96875
0.971875
0.975
0.978125
0.98125
0.984375
0.985938
0.9875
0.989062
0.990625
0.992188
0.992969
0.99375
0.994531
0.995313
0.996094
0.996484
0.996875
0.997266
0.997656
0.998047
0.998242
0.998437
0.998633
0.998828
0.999023
0.999121
0.999219
0.999316
0.999414
0.999512
0.999561
0.999609
0.999658
0.999707
0.999756
0.99978
0.999805
0.999829
0.999854
0.999878
0.99989
0.999902
0.999915
0.999927
0.999939
0.999945
0.999951
0.999957
0.999963
0.999969
0.999973
0.999976
0.999979
0.999982
0.999985
0.999986
0.999988
0.999989
0.999991
0.999992
0.999993
0.999994
0.999995
0.999995
0.999996
0.999997
0.999997
0.999997
0.999998
0.999998
0.999998
0.999998
0.999999
0.999999
0.999999
0.999999
0.999999
0.999999
0.999999
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
```