feat(aws-lambda): Add trace headers to aws_lambda instrumentation

[sam-mosleh]

Description

Automatically inject trace headers in AWS Lambda responses. Fixes #4074

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• By checking existence of those headers

Does This PR Require a Core Repo Change?

• No.

[herin049]

What issue is this trying to solve exactly? Even if you are using API Gateway, Lambda URLs or ALBs, headers should still be propagated normally from client to server. If this change is more about adding the ability to easily retrieve the corresponding trace ID when there is no parent trace to go off of, then there is a tracking issue in the spec for this: open-telemetry/opentelemetry-specification#1355 but from I can tell there is no consensus yet.

If we do decide to implement these changes, I would personally want it to be opt-in only.

[sam-mosleh]

@herin049 You’re correct that trace headers are propagated to the Lambda, but the issue occurs on the response path: when using API Gateway v2 or ALB integrations, the generated trace context is not returned to the client, so the client never receives the trace ID and this is particularly problematic when no incoming trace parent exists. I’ve corrected my implementation to use set_global_response_propagator, making the behavior explicitly opt-in for users who require it.

[herin049]

Ok, I don't really see how this issue is limited to API GW/ALBs though, even if you invoke a lambda function normally with no parent context, you also won't get a reference trace ID back. Either way, approach look OK to me.

[sam-mosleh]

@herin049 API Gateway v1 automagically adds the X-Amzn-Trace-Id to the response headers without the need for Lambdas returning it.

[herin049]

Changes LGTM!