Remove support to carry in the metrics pipeline for OC data

[bogdandrutu]

Split from #1720.

There are some changes that I had to do:

• Remove LabelKey.Description which we don't propagate from OC to OTLP;
• Some places where OC -> OTLP -> OC makes some empty slices to be nil (semantically no change, but when comparing protos this is a diff);
• Some places we did not have MetricDescriptor; metrics without descriptor in OC are dropped.
• Some node attributes are converted to Resource attributes (data are still present).

Updates #841

[codecov]

Codecov Report

Merging #1726 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1726      +/-   ##
==========================================
+ Coverage   92.36%   92.39%   +0.02%     
==========================================
  Files         259      259              
  Lines       18201    18163      -38     
==========================================
- Hits        16812    16781      -31     
+ Misses        976      970       -6     
+ Partials      413      412       -1     

Impacted Files	Coverage Δ
consumer/pdatautil/pdatautil.go	81.48% <100.00%> (-11.17%)	⬇️
...eceiver/prometheusreceiver/internal/transaction.go	95.14% <100.00%> (+0.14%)	⬆️
processor/filterprocessor/filter_processor.go	90.47% <0.00%> (-4.77%)	⬇️
translator/internaldata/oc_to_metrics.go	94.39% <0.00%> (+1.40%)	⬆️
translator/internaldata/resource_to_oc.go	92.59% <0.00%> (+5.55%)	⬆️

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[tigrannajaryan]

Very happy that we started cleaning this up.