otlpmetrichttp: Use go.opentelemetry.io/proto/slim/otlp

[pellared]

Fixes #2579

I tried to do it with as minimal changes as possible while also doing some cleanups (code removal). Here is how I done it:

1. For simple files (without logic) generated via gotmpl , I added protoImportPrefix template variable to control the imports for the proto packages.
2. For complex files (options and test collector) I inlined the source code, removed the unnecessary code for given signal, simplified to have only one option type, and updated to use go.opentelemetry.io/proto/slim/otlp for otlpmetrichttp. I did not want to go to far with the refactoring.
3. In the rest of files in otlpmetrichttp package, I simply replaced go.opentelemetry.io/proto/otlp with go.opentelemetry.io/proto/slim/otlp.

[codecov]

Codecov Report

Attention: Patch coverage is 93.84615% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 85.5%. Comparing base (f885333) to head (d6fd030).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5222     +/-   ##
=======================================
+ Coverage   84.5%   85.5%   +0.9%     
=======================================
  Files        258     258             
  Lines      17348   17030    -318     
=======================================
- Hits       14675   14570    -105     
+ Misses      2362    2152    -210     
+ Partials     311     308      -3     

Files	Coverage Δ
...pmetric/otlpmetricgrpc/internal/otest/collector.go	95.7% <ø> (+73.0%)	⬆️
exporters/otlp/otlpmetric/otlpmetrichttp/client.go	82.4% <ø> (ø)
...porters/otlp/otlpmetric/otlpmetrichttp/exporter.go	87.5% <ø> (ø)
...tlpmetric/otlpmetrichttp/internal/oconf/options.go	98.9% <100.0%> (+6.1%)	⬆️
...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)
...pmetric/otlpmetricgrpc/internal/oconf/envconfig.go	92.0% <83.3%> (-1.0%)	⬇️
... and 2 more

[MrAlias]

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