runtime/metrics: fix panic in readingAllMetric example
medianBucket can return if the total is greater than thresh.
However, if a histogram has no counts, total and thresh
will both be zero and cause panic.
Adding an equal sign to prevent the potential panic.
Fixes #44148
Change-Id: Ifb8a781990f490d142ae7c035b4e01d6a07ae04d
Reviewed-on: https://go-review.googlesource.com/c/go/+/290171
Trust: Ian Lance Taylor <iant@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>