Adding Gatling group response time metric configuration option

[letsrokk]

Summary:

In Gatling HTML reports, requests group response time metric can be either calculated as a cumulated response time of all requests in the group or as a total duration (https://docs.gatling.io/reference/stats/timings/#duration). This is controlled by the Gatling property gatling.charting.useGroupDurationMetric and only affects what value is displayed in the report.

I'd like to introduce a similar configuration option for the Taurus reporter so that we can have matching simulation results in the Taurus console reporter, Blazemeter and Gatling reports

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Quick checklist:

• Description of PR explains the context of change
• Unit tests cover the change, no broken tests
• No static analysis warnings (Codacy etc.)
• Documentation update ('available in the unstable snapshot' warning if necessary)
• Changes file inside site/dat/docs/changes directory, one-line note of change inside

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.77%. Comparing base (b46ec15) to head (26c6c53).
⚠️ Report is 143 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1838      +/-   ##
==========================================
+ Coverage   89.74%   89.77%   +0.03%     
==========================================
  Files          71       71              
  Lines       19531    19548      +17     
==========================================
+ Hits        17527    17548      +21     
+ Misses       2004     2000       -4     

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