Removes the functionality of the Describe in prometheus exporter.

[MadVikingGod]

Describe is called when a collector first registered with prometheus. The intent is for the collector to enumerate all metrics that it might emit in later collects. See the prometheus' Custom Collector Documentation

Because this happens before our SDK is started and because our SDK can dynamically add metrics we should always return no metrics in a Describe() making our collector "unchecked". This PR makes that the case always.

Note: I think this was always the behavior in the new SDK, I don't think there is a way currently to have an SDK that has started, had metrics created, and registered the reader before the collector was registered with prometheus.

Fixes: #3291

[codecov]

Codecov Report

Merging #3342 (d58eb16) into main (0963f59) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3342   +/-   ##
=====================================
  Coverage   77.8%   77.8%           
=====================================
  Files        164     164           
  Lines      11269   11267    -2     
=====================================
+ Hits        8768    8773    +5     
+ Misses      2303    2297    -6     
+ Partials     198     197    -1     

Impacted Files	Coverage Δ
exporters/prometheus/exporter.go	81.4% <100.0%> (+1.4%)	⬆️
sdk/trace/batch_span_processor.go	81.9% <0.0%> (+1.7%)	⬆️