[serverless] Support trace context propagation for ALB target groups with MultiValueHeaders

[quietsato]

What does this PR do?

Adds support for extracting trace context from MultiValueHeaders in ALBTargetGroupRequest

Motivation

Currently, Datadog APM context propagation doesn't work when MultiValueHeaders is enabled in ALB target groups. This PR resolves this issue by allowing trace context propagation even when using MultiValueHeaders configuration.

Describe how to test/QA your changes

• Enable MultiValueHeaders in ALB target group
• Create a Lambda endpoint
• Send a request with trace context headers to the ALB endpoint
• Verify in Datadog's trace view that the trace context is properly propagated from ALB to Lambda

Possible Drawbacks / Trade-offs

• Only the first value from MultiValueHeaders is used, subsequent values are ignored

Additional Notes

• This PR includes test coverage:
  • Unit tests for MultiValueHeadersCarrier
  • Tests for MultiValueHeaders extraction in ALB target groups
  • Tests for MultiValueHeaders support in tag extraction functionality
• Maintains backward compatibility by prioritizing the existing Headers field

https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html#multi-value-headers-request

[bits-bot]

All committers have signed the CLA.

[purple4reina]

I suspect you can probably use https://pkg.go.dev/github.com/DataDog/[email protected]/ddtrace/tracer#HTTPHeadersCarrier which won't require you to reinvent the wheel or do extra allocations.

[quietsato]

Thank you for your suggestion!

I've made the changes in the commit 270eb93 to use HTTPHeadersCarrier from dd-trace-go.
I've removed the multiValueHeadersCarrier function and its associated tests, while ensuring that the final context extraction behavior remains covered by the existing tests.

datadog-agent/pkg/serverless/trace/propagation/extractor_test.go

Lines 544 to 553 in 6eda679

	{
	name: "ALBTargetGroupRequestMultiValueHeaders",
	events: []interface{}{
	events.ALBTargetGroupRequest{
	MultiValueHeaders: toMultiValueHeaders(headersMapAll),
	},
	},
	expCtx: ddTraceContext,
	expNoErr: true,
	},

[purple4reina]

You can do this without an extra allocation by checking event.MultiValueHeaders["Referer"] != nil && len(event.MultiValueHeaders["Referer"]) > 0

[quietsato]

Thank you for the suggestion!
I've implemented this improvement in commit aaaa24f.

[purple4reina]

Thanks so much for this PR. It's a great improvement and we really appreciate your willingness to implement it. I've made just a couple of requests for changes but otherwise this looks great.

[purple4reina]

I reopened this PR as #31542 so the tests will run and we can merge it.