runtime: shift timeHistogram buckets and allow negative durations

Today, timeHistogram, when copied, has the wrong set of counts for the
bucket that should represent (-inf, 0), when in fact it contains [0, 1).
In essence, the buckets are all shifted over by one from where they're
supposed to be.

But this also means that the existence of the overflow bucket is wrong:
the top bucket is supposed to extend to infinity, and what we're really
missing is an underflow bucket to represent the range (-inf, 0).

We could just always zero this bucket and continue ignoring negative
durations, but that likely isn't prudent.

timeHistogram is intended to be used with differences in nanotime, but
depending on how a platform is implemented (or due to a bug in that
platform) it's possible to get a negative duration without having done
anything wrong. We should just be resilient to that and be able to
detect it.

So this change removes the overflow bucket and replaces it with an
underflow bucket, and timeHistogram no longer panics when faced with a
negative duration.

Fixes #43328.
Fixes #43329.

Change-Id: If336425d7d080fd37bf071e18746800e22d38108
Reviewed-on: https://go-review.googlesource.com/c/go/+/279468


Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>