runtime/metrics: mark histogram metrics as cumulative

All the current histogram metrics accumulate counts from program start
to infinity, and can be reasonably used to compute rates (also to
generate windowed distributions).

Change-Id: I5196c59867de34fba41bb8552606fa315460cef9
Reviewed-on: https://go-review.googlesource.com/c/go/+/282633


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>
Reviewed-by: Austin Clements <austin@google.com>