Add alerting system

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It would be great if javamelody had the ability to send alerts based on metrics.

Example: CPU is > 80% for 5 minutes.
Example: Heap is > 80% full for 5 minutes.
Example: Alert on thread pool consumption.
Example: Alert on an error in catalina.

Original issue reported on code.google.com by micah.fi...@gmail.com on 2 Aug 2013 at 1:03

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This would be great and it was already in the TODO list, with alerts sent by 
mail:
https://code.google.com/p/javamelody/wiki/TODO

It would be needed to choose when to send an alert notification by mail (every 
minute is not good, alert on/off seems better).

And it may be difficult to define what are the default thresholds in order to 
work for most applications without false positive.
For example, "GC > 25%" seems better than "Heap is > 80% full for 5 minutes".
"Alert on an error in catalina" seems not good to me (too much noise and 
possibility for the mail server to be blacklisted by peers).

Otherwise, RSS may be an alternative to mail.

Original comment by evernat@free.fr on 6 Aug 2013 at 9:58

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Please keep in mind that such a functionality should be only an addition.
Javamelody - as a small, less-intrusive piece of code - should be able to work 
with such alertings as before. Thanks

Original comment by derj...@googlemail.com on 21 Aug 2013 at 12:29

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Original comment by m.szajow...@gmail.com on 24 Sep 2013 at 3:44

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Original comment by spielmannmartin1990@gmail.com on 9 Jan 2014 at 9:12

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Original comment by stanisla...@gmail.com on 15 Sep 2014 at 2:58

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Original comment by dfernan...@u.uchile.cl on 14 Oct 2014 at 7:44

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Original comment by maxime.d...@gmail.com on 1 Mar 2015 at 4:29