[OTLP] Make `OTEL_EXPORTER_OTLP_CERTIFICATE` independent from mTLS

[Kielek]

The OTEL_EXPORTER_OTLP_CERTIFICATE configuration option does not strictly apply to mTLS.

mTLS is an authentication system in which both the client and server authenticate each other.

The OTEL_EXPORTER_OTLP_CERTIFICATE option should be able to be used independently of an mTLS scenario. Common use case is where a server has a self-signed certificate that has not been verified by a third-party certificate authority. In this scenario you can use the OTEL_EXPORTER_OTLP_CERTIFICATE setting to enable the client to trust the certificate. This is not considered mTLS.

This is not a blocker for this PR since all this code is internal, but for clarity I'd suggest a small refactor reorganizing the code to decouple the the OTEL_EXPORTER_OTLP_CERTIFICATE from any classes with mTLS in their name.

Originally posted by @alanwest in #6343 (comment)

[Kielek]

@sandy2008, FYI

[sandy2008]

@sandy2008, FYI

Hi @Kielek @alanwest,

Thanks for raising this issue. I agree that decoupling OTEL_EXPORTER_OTLP_CERTIFICATE from mTLS-specific classes would improve code clarity. We will share our design proposal before we start to implement.

[sandy2008]

PR: #6777

[sandy2008]

CC: @Kielek @alanwest @rajkumar-rangaraj

[Kielek]

@sandy2008 , we have a tight schedules due to holidays season. I cannot promise that we will be able to handle PR this year.

[sandy2008]

@sandy2008 , we have a tight schedules due to holidays season. I cannot promise that we will be able to handle PR this year.

@Kielek Got it! Thank you :)
Also please take a look at this PR if you have time: open-telemetry/opentelemetry-dotnet-instrumentation#4724

[sandy2008]

@Kielek Thx! Discussed with @sokoide and changed the PR a little bit, please review if you have time.

[sandy2008]

@rajkumar-rangaraj Thank you for merging it, new PR is: #6799