feat: Add add_link to span api/sdk

[dmathieu]

The specification has recently been updated to add the ability to add span links after span creation.

See:

• https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#add-link
• Add a new AddLink() operation to Span. opentelemetry-specification#3678
• Mark the AddLink() operation as stable. opentelemetry-specification#3887

This updates the API and SDK to add this new method.

Note: I am not fully sure how you all are handling additive changes to the API. With Ruby being very permissive with method definitions, I assume shipping everything together is fine.
If you think that could cause an issue (like the API being used with a lower version of the SDK), I'm happy to split this PR into two, so the SDK is shipped first, and the API in another release.

[kaylareopelle]

Thank you for bringing the Ruby project into compliance with this spec update!

I'll defer to the @open-telemetry/ruby-maintainers to your questions about API and SDK releases.

I have a few small documentation comments.

I'm also curious about this line in the spec:

Links added after Span creation may not be considered by Samplers.

Can you tell me how links added using the add_link method are ignored by Samplers?

[dmathieu]

Can you tell me how links added using the add_link method are ignored by Samplers?

SDK samplers, which perform head sampling (not collector samplers, or any other tail sampler) run on span creation. So when add_link is called, the head sampling decision has already been made, and is not reconsidered with the new link.

[kaylareopelle]

SDK samplers, which perform head sampling (not collector samplers, or any other tail sampler) run on span creation. ...

Thank you, @dmathieu. This connects the dots!