Support default histogram selection in OTLP exporter

[MrAlias]

Resolve #4422

If a user sets the OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION to base2_exponential_bucket_histogram (case-insensitive) the default base-2 exponential histogram should be used for the exporter selector in both OTLP metric exporters.

[codecov]

Codecov Report

Merging #4437 (5734939) into main (af1d928) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4437   +/-   ##
=====================================
  Coverage   78.8%   78.8%           
=====================================
  Files        253     253           
  Lines      20630   20668   +38     
=====================================
+ Hits       16267   16307   +40     
+ Misses      4014    4012    -2     
  Partials     349     349           

Files Changed	Coverage Δ
...pmetric/otlpmetricgrpc/internal/oconf/envconfig.go	93.1% <100.0%> (+1.1%)	⬆️
...pmetric/otlpmetrichttp/internal/oconf/envconfig.go	93.1% <100.0%> (+1.1%)	⬆️

... and 1 file with indirect coverage changes

[evantorrie]

LGTM.

I haven't been reviewing lately, but see that we're using gotmpl for this repetitive code between otlpmetric{grpc,http}. Looks like this is the outcome of #3846.

I think many of the language SDKs have similar issues.