runtime: simplify histogram buckets considerably

There was an off-by-one error in the time histogram buckets calculation
that caused the linear sub-buckets distances to be off by 2x.

The fix was trivial, but in writing tests I realized there was a much
simpler way to express the calculation for the histogram buckets, and
took the opportunity to do that here. The new bucket calculation also
fixes the bug.

Fixes #50732.

Change-Id: Idae89986de1c415ee4e148f778e0e101ca003ade
Reviewed-on: https://go-review.googlesource.com/c/go/+/380094


Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>