autoexport: OTLP exporter defaults to http/protobuf protocol instead of grpc

[pellared]

Why

https://github.com/open-telemetry/opentelemetry-specification/blob/v1.23.0/specification/protocol/exporter.md?plain=1#L152-L155

If no configuration is provided the default transport SHOULD be http/protobuf
unless SDKs have good reasons to choose grpc as the default (e.g. for backward
compatibility reasons when grpc was already the default in a stable SDK
release).

Also a bugfix https://github.com/open-telemetry/opentelemetry-specification/blob/v1.23.0/specification/configuration/sdk-environment-variables.md?plain=1#L19

The SDK MUST interpret an empty value of an environment variable the same way as when the variable is unset.

What

OTLP exporter with http/protobuf protocol is created by default.

OTEL_EXPORTER_OTLP_PROTOCOL="" is handled the same way as if OTEL_EXPORTER_OTLP_PROTOCOL was unset.

Testing

Unfortunately there is no easy way to test what OTLP exporter is returned (grpc vs http). My only idea was to use reflect, but I am not sure if it is not an overkill.

I decided to add the tests anyway: 8a1cf4e

I created #4102 to improve the documentation.

[codecov]

Codecov Report

Merging #4100 (8a1cf4e) into main (33c3a8f) will increase coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4100     +/-   ##
=======================================
+ Coverage   79.1%   79.3%   +0.1%     
=======================================
  Files        165     165             
  Lines      10318   10318             
=======================================
+ Hits        8170    8183     +13     
+ Misses      2010    2000     -10     
+ Partials     138     135      -3     

Files Changed	Coverage Δ
exporters/autoexport/exporter.go	85.0% <100.0%> (+7.5%)	⬆️
exporters/autoexport/registry.go	100.0% <100.0%> (+17.5%)	⬆️

... and 2 files with indirect coverage changes