Baggage propagator does not handle multiple headers

[samuong]

Description

According to section 3 of the W3C Baggage spec:

Multiple baggage headers are allowed. Values can be combined in a single header according to RFC 7230.

Section 3.4 gives this example:

Context might be split into multiple headers:

baggage: userId=alice
baggage: serverNode=DF%2028,isProduction=false

But the baggage propagator's Extract() method ends up using https://pkg.go.dev/net/http#Header.Get rather than https://pkg.go.dev/net/http#Header.Values, so it is only able to get a single header value. For the example above (which I've reproduced in the test below) it only sees the key-value pair for userId=alice, and not the other two.

I'm not sure if there are other, better, ways to do this but in my test I am using a HeaderCarrier to adapt http.Header to a TextMapCarrier. This only has a Get() method in the interface, no Values():

opentelemetry-go/propagation/propagation.go

Lines 11 to 30 in 7846383

	// TextMapCarrier is the storage medium used by a TextMapPropagator.
	type TextMapCarrier interface {
	// DO NOT CHANGE: any modification will not be backwards compatible and
	// must never be done outside of a new major release.
	// Get returns the value associated with the passed key.
	Get(key string) string
	// DO NOT CHANGE: any modification will not be backwards compatible and
	// must never be done outside of a new major release.
	// Set stores the key-value pair.
	Set(key string, value string)
	// DO NOT CHANGE: any modification will not be backwards compatible and
	// must never be done outside of a new major release.
	// Keys lists the keys stored in this carrier.
	Keys() []string
	// DO NOT CHANGE: any modification will not be backwards compatible and
	// must never be done outside of a new major release.
	}

If it's important to avoid changing the TextMapCarrier interface, then perhaps the HeaderCarrier.Get() method could join together multiple header values into a single comma-separated string?

Environment

• OS: Fedora 41
• Architecture: amd64
• Go Version: 1.23.4
• opentelemetry-go version: v1.33.0

Steps To Reproduce

1. Run https://goplay.tools/snippet/1DWw9PC4tHr
2. See error

Expected behavior

I expected all the tests to pass, and for the values of userId, serverNode and isProduction to be alice, DF%2028 and false, respectively.

[pellared]

Related prototype: #5973