runtime/metrics: specify that bucket counts increase monotonically for histogram...

runtime/metrics: specify that bucket counts increase monotonically for histogram metrics

Make it explicit in the documentation that the histogram metrics
are cumulative (i.e. each bucket count increases monotonically).

Change-Id: I89119ba816ac46a63f36e607e695fad3695057ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/487315
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
Reviewed-by: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
TryBot-Result: Gopher Robot <gobot@golang.org>