[Test] Use slim proto module

[pellared]

Tests open-telemetry/opentelemetry-proto-go#155

No more gRPC dependencies in:

• https://github.com/pellared/opentelemetry-go/blob/slim-poc/exporters/otlp/otlpmetric/otlpmetrichttp/go.mod
• https://github.com/pellared/opentelemetry-go/blob/slim-poc/exporters/otlp/otlptrace/otlptracehttp/go.mod

PS. I have removed the OTLP gRPC becasue this PR is just for sake of testing. The actual changes would probably get rid of the .go.tmpl code as it becomes more of a burden than benefit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.9%. Comparing base (da2949b) to head (2a4ff5d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5031     +/-   ##
=======================================
+ Coverage   83.4%   83.9%   +0.5%     
=======================================
  Files        238     210     -28     
  Lines      15754   13682   -2072     
=======================================
- Hits       13146   11487   -1659     
+ Misses      2318    1943    -375     
+ Partials     290     252     -38     

Files	Coverage Δ
exporters/otlp/otlpmetric/otlpmetrichttp/client.go	82.0% <ø> (ø)
...porters/otlp/otlpmetric/otlpmetrichttp/exporter.go	87.5% <ø> (ø)
...tlpmetric/otlpmetrichttp/internal/oconf/options.go	88.6% <ø> (-3.9%)	⬇️
...otlpmetric/otlpmetrichttp/internal/otest/client.go	97.7% <ø> (ø)
...pmetric/otlpmetrichttp/internal/otest/collector.go	72.3% <ø> (+11.7%)	⬆️
...ric/otlpmetrichttp/internal/transform/attribute.go	100.0% <ø> (ø)
...pmetric/otlpmetrichttp/internal/transform/error.go	100.0% <ø> (ø)
...ic/otlpmetrichttp/internal/transform/metricdata.go	100.0% <ø> (ø)
...tlp/otlptrace/internal/tracetransform/attribute.go	100.0% <ø> (ø)
...lptrace/internal/tracetransform/instrumentation.go	100.0% <ø> (ø)
... and 6 more

... and 9 files with indirect coverage changes