Add a new dropped_iterations metric

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[codecov-commenter]

Codecov Report

Merging #1529 into new-schedulers will increase coverage by 0.02%.
The diff coverage is 92.59%.

@@                Coverage Diff                 @@
##           new-schedulers    #1529      +/-   ##
==================================================
+ Coverage           77.29%   77.32%   +0.02%     
==================================================
  Files                 162      162              
  Lines               13095    13127      +32     
==================================================
+ Hits                10122    10150      +28     
- Misses               2455     2457       +2     
- Partials              518      520       +2     

Impacted Files	Coverage Δ
lib/executor/base_executor.go	83.33% <42.85%> (-16.67%)	⬇️
js/runner.go	83.10% <100.00%> (+0.11%)	⬆️
lib/executor/constant_arrival_rate.go	96.75% <100.00%> (+0.09%)	⬆️
lib/executor/constant_vus.go	96.73% <100.00%> (ø)
lib/executor/per_vu_iterations.go	97.27% <100.00%> (+0.12%)	⬆️
lib/executor/ramping_arrival_rate.go	96.72% <100.00%> (+0.07%)	⬆️
lib/executor/ramping_vus.go	94.23% <100.00%> (ø)
lib/executor/shared_iterations.go	97.69% <100.00%> (+0.11%)	⬆️
lib/testutils/minirunner/minirunner.go	78.57% <100.00%> (-2.92%)	⬇️
... and 1 more

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[imiric]

LGTM, just a couple of questions.