Add otlpmetricgrpc.Expoter and otlpmetrichttp.Exporter

[pellared]

Fixes #4247
Fixes #4248

[codecov]

Codecov Report

Merging #4272 (1f63deb) into main (41cd6a1) will decrease coverage by 0.1%.
The diff coverage is 51.2%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4272     +/-   ##
=======================================
- Coverage   83.5%   83.5%   -0.1%     
=======================================
  Files        181     183      +2     
  Lines      14189   14211     +22     
=======================================
+ Hits       11861   11875     +14     
- Misses      2100    2108      +8     
  Partials     228     228             

Impacted Files	Coverage Δ
exporters/otlp/otlpmetric/otlpmetricgrpc/client.go	87.7% <ø> (+2.0%)	⬆️
exporters/otlp/otlpmetric/otlpmetrichttp/client.go	77.5% <ø> (+0.9%)	⬆️
...porters/otlp/otlpmetric/otlpmetricgrpc/exporter.go	47.0% <47.0%> (ø)
...porters/otlp/otlpmetric/otlpmetrichttp/exporter.go	47.0% <47.0%> (ø)
exporters/otlp/otlpmetric/internal/exporter.go	67.2% <71.4%> (ø)

... and 2 files with indirect coverage changes