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## Problem We can't easily tell how far the state of shards is from their AZ preferences. This can be a cause of performance issues, so it's important for diagnosability that we can tell easily if there are significant numbers of shards that aren't running in their preferred AZ. Related: https://github.com/neondatabase/cloud/issues/15413 ## Summary of changes - In reconcile_all, count shards that are scheduled into the wrong AZ (if they have a preference), and publish it as a prometheus gauge. - Also calculate a statistic for how many shards wanted to reconcile but couldn't. This is clearly a lazy calculation: reconcile all only runs periodically. But that's okay: shards in the wrong AZ is something that only matters if it stays that way for some period of time.