Improve how we do logging

[heyman]

PR for #1285. Does the following:

• We no longer wrap sys.stdout and sys.stderr (yay!). print() statements in locust scripts will no longer automatically go into the log. If someone wants to add entries to the log they can use the root logger instead (logging.info(…)), or any custom logger named "locust.*".
• The "console_logger" logger has been removed. For printing stats to the console we now use a logger called "locust.stats_logger". For printing output when using flags such as --show-task-ratio or -l we now use print() statements instead.
• We now set up an exception handler that logs any unhandled exception, for every greenlet we spawn (using Greenlet.link_exception) unless they already have their own exception handler (e.g. Locust User greenlets).
• Only allow valid --loglevel values.
• Better tests.

[codecov]

Codecov Report

Merging #1325 into master will decrease coverage by 0.21%.
The diff coverage is 46.66%.

@@            Coverage Diff             @@
##           master    #1325      +/-   ##
==========================================
- Coverage   80.99%   80.77%   -0.22%     
==========================================
  Files          24       24              
  Lines        2199     2190       -9     
  Branches      333      334       +1     
==========================================
- Hits         1781     1769      -12     
- Misses        331      334       +3     
  Partials       87       87              

Impacted Files	Coverage Δ
locust/inspectlocust.py	71.87% <0.00%> (-0.86%)	⬇️
locust/main.py	21.57% <5.88%> (-0.71%)	⬇️
locust/log.py	56.25% <50.00%> (-7.64%)	⬇️
locust/runners.py	78.78% <100.00%> (-0.21%)	⬇️
locust/stats.py	88.73% <100.00%> (+0.02%)	⬆️
locust/web.py	89.65% <0.00%> (-1.15%)	⬇️
locust/core.py	98.17% <0.00%> (+0.45%)	⬆️

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[cyberw]

Looks good! But is there some reason plain print() statements cant also work?

[heyman]

But is there some reason plain print() statements cant also work?

Do you mean why print() statements won't automatically go into the log file? That was an effect of us wrapping sys.stdout which seems like a strange and invasive behaviour. Print statements still work, it's just that they now does what one would expect them to do in python (just print to the console).

[cyberw]

But is there some reason plain print() statements cant also work?

Do you mean why print() statements won't automatically go into the log file? That was an effect of us wrapping sys.stdout which seems like a strange and invasive behaviour. Print statements still work, it's just that they now does what one would expect them to do in python (just print to the console).

Ah, I misunderstood you and thought you meant that print statements no longer work for printing to stdout :)

+1