Creating watcher for index rate to alert in case of excessive logging

How would you create a watcher to email alert you when the index rate for an index exceeds something stupid.

Use Case:
When everything is fine we don't log too much to this index but if one of our apps has a problem it will start smashing the index as fast as it can with errors. Up to 1,000 per second easily. So we really need a watcher to email us and tell us when it exceeds 500 per second or so.

So how would you write a query to return the index rate?