Generalize param mapping and allow setting arbitrary metadata in statistics-mtg2 action

[tweska]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 59 lines in your changes missing coverage. Please review.

Please upload report for BASE (feature/statistics-mtg2@29c0fc5). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...o/action/statistics-mtg2/StatisticsParamMapping.cc	0.00%	30 Missing ⚠️
...io/action/statistics-mtg2/cfg/StatisticsOptions.cc	0.00%	14 Missing ⚠️
src/multio/action/statistics-mtg2/Statistics.cc	0.00%	8 Missing ⚠️
...io/action/statistics-mtg2/StatisticsParamMapping.h	0.00%	7 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##             feature/statistics-mtg2     #104   +/-   ##
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  Coverage                           ?   46.90%           
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  Files                              ?      260           
  Lines                              ?    17395           
  Branches                           ?     1423           
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  Hits                               ?     8159           
  Misses                             ?     9236           
  Partials                           ?        0           

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[dsarmany]

See my comments. Let's start with addressing those and then we can have another go.

[dsarmany]

I wouldn't create a singleton for this just yet. We may do that later, if need be. Also, there is very specific pattern to follow when we do create a singleton. See Aggregation or Mask, for example.