runtime/metrics: fix panic in readingAllMetric example
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

CAFxX/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go