Add `period` (sampling) information in pprof web viewer.

Thanks for the amazing project!

It would be amazing to have information about the period (so sampling) of the profiles we are analysing. I would expect it to see in pop up when clicking on the right menu item with the file name and sample type, but it's not there:


### Proposal

* Add field `Sampling:` to the mentioned pop-up based on `period` and `period_type`. E.g. for my profile above it would be `Sampling: 512 KB (space)`.

### Question

Am I right that the only way to discover the period right now is through `-raw`? e.g.:

```bash
go tool pprof -raw ~/Downloads/heap | head
PeriodType: space bytes
Period: 524288
Time: 2022-07-13 21:32:01.407524692 +0200 CEST
Samples:
alloc_objects/count alloc_space/bytes inuse_objects/count inuse_space/bytes
341 525056 0 0: 1 2 3 4 5 6 7 8 9 10 11
bytes:[1536]
10923 524312 0 0: 12 13 14 14 15 16 17
bytes:[48]
8192 524320 0 0: 18 19 20 21 22 23 24 10 11
```