exporters/autoexport: add support for signal-specific protocols environment variables

[thomasgouveia]

This PR adds support for signal-specific environment variables to configure the OTLP protocol used with the exporter.

The following environment variables have been added to configure signal-specific protocols:

• OTEL_EXPORTER_OTLP_TRACES_PROTOCOL
• OTEL_EXPORTER_OTLP_LOGS_PROTOCOL
• OTEL_EXPORTER_OTLP_METRICS_PROTOCOL

The package will first attempt to load the protocol for a signal from the corresponding environment variable. If it is not set or is empty, it will try to determine the protocol from OTEL_EXPORTER_OTLP_PROTOCOL. If this is also not defined or is empty, it will fall back to http/protobuf.

Note for reviewers: As you'll see in the code, I use my own env package (MIT License) to facilitate environment variable management. If needed, I can backport the used functions into the code here and remove the dependency. Let me know.

Closes #5807

[pellared]

If needed, I can backport the used functions into the code here and remove the dependency.

Please do that.

Also, please update the changelog.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.5%. Comparing base (fbc3302) to head (9a444bf).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5816   +/-   ##
=====================================
  Coverage   64.5%   64.5%           
=====================================
  Files        200     200           
  Lines      12563   12569    +6     
=====================================
+ Hits        8112    8118    +6     
  Misses      4215    4215           
  Partials     236     236           

Files	Coverage Δ
exporters/autoexport/logs.go	100.0% <100.0%> (ø)
exporters/autoexport/metrics.go	78.9% <100.0%> (+0.3%)	⬆️
exporters/autoexport/spans.go	92.3% <100.0%> (+0.6%)	⬆️

[dmathieu]

How about using a similar approach to the OTLP exporter?
https://github.com/open-telemetry/opentelemetry-go/blob/cda5094cdd5746c995b30edbb16da42a1d83442e/exporters/otlp/otlptrace/otlptracehttp/internal/otlpconfig/envconfig.go#L46

[thomasgouveia]

How about using a similar approach to the OTLP exporter?

@dmathieu seems to be a good solution to have something centralized. Ideally, you'd like to load all the configurations into a struct and use this struct to access the configuration once loaded? But as you said, it is a simple condition. This solution seems to be appropriate if we need to add more configuration in the future.

[pellared]

@thomasgouveia, you do not need to rebase and force-push. Regular merge commits are fine. Also it also makes reviewing easier (at least for me) as the UI can show the diff between commits.

From https://github.com/open-telemetry/opentelemetry-go/blob/main/CONTRIBUTING.md#how-to-send-pull-requests:

Avoid rebasing and force-pushing to your branch to facilitate reviewing the pull request. Rewriting Git history makes it difficult to keep track of iterations during code review. All pull requests are squashed to a single commit upon merge to main.

[thomasgouveia]

@pellared sorry for that, I'll add regular commits in future PRs!