Baggage API: Expand the API for accessing metadata

[pellared]

What are you trying to achieve?

The OTel spec does not even have the "Members" and "Property" distinction reflecting W3C definitions. I would even say that the OTel spec "properties" reflects "members" in W3C which makes it very confusing.

If for some reason it is intended to NOT handle properties (also known as metadata) like in W3C then it would be good to call it out in the specification.

Additional context.

Related PR: open-telemetry/opentelemetry-go#5132 (comment)

[pellared]

The Baggage API in Python, .NET, C++ does NOT support "properties" (also known as metadata).

The Baggage API in Java, Go, Rust supports "properties" (also known as metadata).

[pellared]

CC @open-telemetry/rust-approvers

[jack-berg]

Discussed in the 4/2/24 Spec SIG meeting and determined we ought to improve the spec. We currently do mention properties / metadata, but only for the Set Value operation, and we only mention that it is an opaque value. We should:

• Expand the API to add getters for accessing properties / metadata
• Add helper functions so that properties / metadata isn't opaque. I.e. give helpers for reading / writing multiple entries.

[pellared]

CC @open-telemetry/dotnet-approvers @open-telemetry/python-approvers @open-telemetry/cpp-approvers (as the API does not even accept the metadata)