system check: total cpu usage test

[Tildeslash]

add total cpu usage test which will allow to trigger action regardless of user/system/wait category ... currently we support only fine-grained cpu usage test (too complicated for general use case where the user is interested only about total system cpu usage)

[Alain Mouette]

+1... yes, this will be very usefull. ... and something more: highest CPU Core usage. This is very important for single threaded applications like Node.js that can overload just one Core!

[Former User]

Why is this on hold? This is very basic core functionality. You need to knock this one out!

[Tildeslash]

fixed Issue #167 ... add support for simple system-wide cpu usage check (total of usr% + sys% + wait%)

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