6660e3c9a0
This adds a section to the output like this:
```
### Percentile metrics
p50 p90 p95 count > 2s
ms per completion 4 6 7 0
```
Bug: https://github.com/dart-lang/sdk/issues/48788
Change-Id: I868580324b3bc83605aa6466ffd5799625d1a9f3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/240941
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>