Decouple otlpmetricgrpc and otlpmetrichttp from otlpmetric

[pellared]

I think this is the only user-facing API change that it would be good to do before making OTLP metrics exporters stable.

[codecov]

Codecov Report

Merging #4660 (a2d938e) into main (cab95bc) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #4660   +/-   ##
=====================================
  Coverage   81.3%   81.4%           
=====================================
  Files        223     225    +2     
  Lines      17721   17725    +4     
=====================================
+ Hits       14423   14429    +6     
+ Misses      2998    2996    -2     
  Partials     300     300           

Files	Coverage Δ
exporters/otlp/otlpmetric/otlpmetricgrpc/client.go	91.0% <100.0%> (+0.3%)	⬆️
...tlpmetric/otlpmetricgrpc/internal/oconf/options.go	87.6% <ø> (-0.2%)	⬇️
...xporters/otlp/otlpmetric/otlpmetricgrpc/version.go	100.0% <100.0%> (ø)
exporters/otlp/otlpmetric/otlpmetrichttp/client.go	78.7% <100.0%> (ø)
...tlpmetric/otlpmetrichttp/internal/oconf/options.go	90.9% <ø> (-0.2%)	⬇️
...xporters/otlp/otlpmetric/otlpmetrichttp/version.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes