`tracing` field missing from Lambda extension INVOKE event

[ramosbugs]

The AWS Lambda Extensions API seems to set the tracing field in the INVOKE event for Lambda extensions even if X-Ray active tracing is disabled for the Lambda function and the requesting service (e.g., API Gateway) doesn't provide tracing headers. It's unclear where the trace ID originates, but I confirmed that it's present even when X-Ray is disabled. When I searched the trace ID in the X-Ray console, it didn't find anything, suggesting that the AWS Lambda service generates a new trace ID when none is provided.

However, the emulator omits this field when the request doesn't include an X-Amzn-Trace-Id header:

aws-lambda-runtime-interface-emulator/lambda/rapi/rendering/rendering.go

Line 144 in 2057ada

Tracing: model.NewXRayTracing(req.TraceID),

aws-lambda-runtime-interface-emulator/lambda/rapi/model/tracing.go

Lines 30 to 33 in 2057ada

	func NewXRayTracing(value string) *Tracing {
	if len(value) == 0 {
	return nil
	}

This behavior breaks compatibility with the AWS Lambda Rust Runtime, which expects the tracing field (cc @calavera):
https://github.com/awslabs/aws-lambda-rust-runtime/blob/ab3b809d63fad05369596d17ad4fc449f7035e5e/lambda-extension/src/events.rs#L23

When locally testing Lambda extensions using the Rust runtime and the AWS SAM CLI's sam local command, extensions fail with a "missing field tracing" error. The same extensions work fine in production.

Since this incompatibility is due to a behavioral difference between the AWS Lambda service in production and the emulator, I think it's ultimately a bug in the emulator, rather than the Rust runtime being overly strict.

I'd suggest changing the NewXRayTracing function to generate a trace ID when none is provided, rather than returning nil.