Help operators to test their bridges in China

I saw some engagement with the new metrics "blocklist" info. I think having that info displayed for other countries like China would be good for the bridge operator community, as many of them don't know that their bridges are blocked.

As this would require some integration in rdsys/metrics/probetest and more work for the AC team, we could start small. @meskio and @shelikhoo suggested of writing a short howto to be published in the Support portal to help operators to test manually their bridge if it's blocked in China.