Move otlpmetrics Client to an internal package.

[MadVikingGod]

Resolves #3455

This was discussed in the SIG meeting. We want to move the exporters/otlp/otlpmetric.Client interface into an internal package. This allows for the current common logic to be shared between otlpmetrichttp and otlpmetricgrpc, but allows us to expose this in the future if needed.

Users of otlpmetric.New() will need to use the otlpmetric[http|grpc].New() directly.

[MrAlias]

LGTM when minor issues resolved.

[codecov]

Codecov Report

Merging #3486 (470b506) into main (c4333a9) will not change coverage.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3486   +/-   ##
=====================================
  Coverage   77.8%   77.8%           
=====================================
  Files        165     165           
  Lines      11614   11614           
=====================================
  Hits        9046    9046           
  Misses      2372    2372           
  Partials     196     196           

Impacted Files	Coverage Δ
exporters/otlp/otlpmetric/internal/exporter.go	67.2% <ø> (ø)
exporters/otlp/otlpmetric/internal/otest/client.go	97.1% <ø> (ø)
exporters/otlp/otlpmetric/otlpmetricgrpc/client.go	85.7% <100.0%> (ø)
exporters/otlp/otlpmetric/otlpmetrichttp/client.go	76.6% <100.0%> (ø)