Improve percentiles computation with HDR histogram

This commit also introduces new fields in the Metrics.Latencies struct.
* Min
* P999
However, it seems to work with `sigfigs=3`:
```
Latencies [mean, min, 50, 95, 99, 999, max] 471.728us, 186.884us, 393.215us, 1.966079ms, 3.014655ms, 4.849663ms, 5.837663ms
```