Propagation - Extract handles multiple values on carrier using same key

[jamesmoessis]

Fixes #433

Discussed in 11/5/24 Spec SIG, and prototyped in Java and Go.

Changes

Adds GetAll method to Getter, allowing for the retrieval of multiple keys for the same value. For example, an HTTP request may have multiple baggage headers.

As written in the changes, the implementation of GetAll is strictly optional. This is for two reasons:

1. Backwards compatibility with existing types/interfaces
2. Carriers do not necessarily support returning multiple values for a single key

Links to Prototypes

This includes implementations of GetAll() in Java and Go, as well as using the method in the W3C Baggage Propagators (multiple baggage headers are allowed, spec reference).

• Java: Extends TextMapGetter with GetAll() method, implement usage in W3CBaggagePropagator opentelemetry-java#6852
• Go: Prototypes proposed spec changes to allow extraction of multiple header values opentelemetry-go#5973

• Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes

[jack-berg]

Couple of minor comments

[pellared]

The Go prototype does not have benchmarks that would showcase that there is no considerable performance degradation in hot-path. Without the benchmarks the OTel Go maintainers are not able to guarantee that we OTel Go is going to support GetAll.

[jack-berg]

The Go prototype does not have benchmarks that would showcase that there is no considerable performance degradation in hot-path. Without the benchmarks the OTel Go maintainers are not able to guarantee that we OTel Go is going to support GetAll.

Only propagators that have the possibility of values being contained in multiple headers would need to call this new method, and be concerned about the perf overhead. The only propagator I know that fits this is the w3c baggage propagator. And the perf hit (if any) seems to be essential complexity required by the w3c specification.

[pellared]

The only propagator I know that fits this is the w3c baggage propagator

W3C TraceContext as well (sic!):

Tracestate MAY be sent or received as multiple header fields.

And the perf hit (if any) seems to be essential complexity required by the w3c specification.

I mean a performance hit even in a scenario that everything is in one header. The performance hit we are worried about it related to Go how it handles type assertions. People may start to complain that in most common scenario (when 1 header) the performance is worse. See open-telemetry/opentelemetry-go#5973 (comment).

[jamesmoessis]

@pellared do you still have performance concern after you found no extra allocations? From your comment: open-telemetry/opentelemetry-go#5973 (comment)

I also think there's something to be said about the value of correctness outweighing potential minor performance concerns in this case.

[pellared]

@pellared do you still have performance concern after you found no extra allocations? From your comment: open-telemetry/opentelemetry-go#5973 (comment)

No. Approving. Thanks for your contribution 🏅

[jack-berg]

Build is failing because of unrelated broken link, fixed here: #4314