Add example that manually adds stats entries

[heyman]

This PR adds an example with a manual_report() decorator/context manager that can be used to easily measure the time of functions/code blocks and manually report them as stats entries to the Locust statistics.

I wrote up the manual_report() functionality as an example for a user in the Locust Slack, and I think it turned out pretty.

We could also consider actually making manual_report part of Locust's API. For example underlocust.contrib.report.manual_report. Though I'm not sure it's useful enough to warrant including it with locust itself?

Usage example as a context manager:

with manual_report("stats entry name"):
    # Run time of this block will be reported under a stats entry names "stats entry name" (of type "manual")
    # If an Exception is raised, it'll be reported as a failure

Usage as a decorator:

@task
@manual_report
def my_task(self):
   # The run time of this task will be reported under a stats entry named "my_task" (of type "manual")
   # If an Exception is raised, it'll be reported as a failure

Even if we don't want to add it udner locust.contrib.report.manual_report I think it can be a useful example.

[codecov]

Codecov Report

Merging #1645 (a422c7c) into master (a9eae19) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1645      +/-   ##
==========================================
+ Coverage   80.79%   80.82%   +0.03%     
==========================================
  Files          30       30              
  Lines        2676     2676              
  Branches      411      411              
==========================================
+ Hits         2162     2163       +1     
  Misses        419      419              
+ Partials       95       94       -1     

Impacted Files	Coverage Δ
locust/stats.py	88.82% <0.00%> (+0.19%)	⬆️

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[cyberw]

LGTM! When you make PRs and I approve them, do you want to click the merge button yourself or should I do it? :)

[heyman]

Feel free to merge them unless they are drafts or I've explicitly said that they aren't ready to be merged for some reason :).