Update aws lambda spec to remove X-Ray Env propagation

[tylerbenson]

Per discussion in the FAAS SIG, we decided that the aws x-ray environment variable should be moved to a span link to avoid interfering with the configured propagators.

We also decided that EventToCarrier should be specified in the spec to improve cross language instrumentation consistency.

I removed EventToCarrier from this PR to focus on the x-ray propagation. We can revisit in a separate PR.

Fixes #3060

[tylerbenson]

@Aneurysm9 note: I pulled the section about EventToCarrier as it was causing some confusion. We should discuss further and put in a separate PR for it.

[cartersocha]

fyi @tedsuo we have a spec reviewer block here

[cartersocha]

Approved by AWS Lambdas SME's - @Aneurysm9

[cartersocha]

No GCP or Azure SME input required

[tigrannajaryan]

This has enough approvals but merging is blocked because there are unresolved conversations.

[tylerbenson]

@tigrannajaryan thanks for the call-out. I've resolved the remaining conversation. Should be good to go!

[carlosalberto]

@tyler Forgot to mention we need a CHANGELOG entry as well 😓

[wangzlei]

1. This spec changes might be against with Span links spec and Messaging system. FYI the ambient case in https://github.com/open-telemetry/oteps/blob/main/text/trace/0220-messaging-semantic-conventions-span-structure.md#user-content-instrumenting-pull-based-scenarios. If user's Lambda function is invoked by AWS Lambda, AWS Lambda should be the parent of user Lambda function.
2. AWS Lambda is working on the passthrough mode fix and would be released soon. After this fix, if user disables tracing for a root Lambda, the environment does not have trace header with sample flag 0.

May I know the deeper reason why changing parent-child to be links?

[tylerbenson]

@wangzlei This PR is probably not the best place to have this discussion. Can I suggest you open a new issue or join the FAAS SIG meeting?

The quick summary is that the prior behavior would break traces for other vendors when x-ray was enabled because the x-ray internal spans were not available.

[carlosalberto]

@wangzlei As Tyler said, please open an issue in https://github.com/open-telemetry/semantic-conventions (and maybe join the FaaS call? We meet every Thursday at 8am Pacific)