Added user-defined wait_function to locust and TaskSet

[ps-george]

Most real-world inter-arrival times are exponentially distributed so it would be nice to be able to model that using locust. I've added the possibility for a user-defined wait_function, so any desired inter-arrival time can be modelled. Added some examples of exponential wait times I'd like to be able to model. If wait_function is not set, it will default to exactly as before, i.e. random.randint(self.min_wait,self.max_wait)

Change-log needs updating if merging this.

[codecov]

Codecov Report

Merging #785 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #785      +/-   ##
==========================================
+ Coverage   64.99%   65.26%   +0.26%     
==========================================
  Files          14       14              
  Lines        1417     1422       +5     
  Branches      222      224       +2     
==========================================
+ Hits          921      928       +7     
+ Misses        444      443       -1     
+ Partials       52       51       -1

Impacted Files	Coverage Δ
locust/core.py	79.5% <100%> (+0.52%)	⬆️
locust/clients.py	89.88% <0%> (+2.24%)	⬆️

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[heyman]

Thanks for the PR!

This is currently possible to achieve by overriding the wait() method in the TaskSet class. However I think this is a nicer API. This would also resolve #18 :).

I'm +1

Does anyone else has thoughts on this?

[aldenpeterson-wf]

+1 as well. I wish it was easier to figure out what sections of docs to update short of having them all memorized.

Also this is a great PR example too. A small, concise, clear feature request with a good API... 🏆

[cgoldberg]

thanks for including an example too!

[ps-george]

Thanks for the fast and welcoming response!

[aldenpeterson-wf]

@ps-george realistically it's easy in a PR like this ;-)