Revert "[chore] Refactor mdatagen unmarshaling to use less custom Unmarshalers"

[atoulme]

Reverts #9760

Trying to make contrib happy. We will be back with a unit test covering contrib's usage of mdatagen.

[codecov]

Codecov Report

Attention: Patch coverage is 86.36364% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 90.98%. Comparing base (8990be3) to head (28ece75).

Files	Patch %	Lines
cmd/mdatagen/metricdata.go	86.36%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9773      +/-   ##
==========================================
- Coverage   90.99%   90.98%   -0.02%     
==========================================
  Files         353      353              
  Lines       18636    18648      +12     
==========================================
+ Hits        16958    16967       +9     
- Misses       1352     1354       +2     
- Partials      326      327       +1     

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[dmitryax]

We need to add the failing use case to https://github.com/open-telemetry/opentelemetry-collector/tree/main/cmd/mdatagen/internal/samplereceiver before trying to bring this back.

[atoulme]

I'm on this @dmitryax