Move X-Ray Env Variable propagation to span link instead of parent

[tylerbenson]

open-telemetry/opentelemetry-specification#3166

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.

Also related to #5167.

[laurit]

@tylerbenson Considering that all backends don't support span links would it make sense to make this configurable so vendors/users could revert to previous behaviour. WDYT?

[tylerbenson]

@laurit The only vendor that should care about the current behavior is Amazon/XRay. For everyone else, this behavior interferes with the regular trace propagation and results in broken traces. Moving it to a span link solves the broken propagation and maintains the information for x-ray customers.

[laurit]

@tylerbenson I understand what you are trying to do. I haven't used aws lambdas myself so perhaps I'm misunderstanding something. Let me rephrase my question. Our aws sdk instrumentation https://github.com/open-telemetry/opentelemetry-java-instrumentation/tree/main/instrumentation/aws-sdk/aws-sdk-2.2/library#trace-propagation currently uses only x-ray headers for propagation. If a request sent from aws sdk triggers a lambda that should only have the x-ray header. Previously parent context was set based on that x-ray header, now a span link is created instead. Is it possible that on a backend that does not support span links previously you got a complete trace and now after these changes you'll get a broken trace (because of missing support for span links)?