Add add_link API

[lalitb]

Changes

The API is already supported as experimental in specification:
https://github.com/open-telemetry/opentelemetry-specification/blob/v1.29.0/specification/trace/api.md#add-link

And planned to make stable soon - open-telemetry/opentelemetry-specification#3865. So not added under experimental flag.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 61.53846% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 68.8%. Comparing base (13c9dc5) to head (5cbc2c3).

Files	Patch %	Lines
opentelemetry-sdk/src/trace/span.rs	80.4%	9 Missing ⚠️
opentelemetry/src/trace/mod.rs	33.3%	8 Missing ⚠️
opentelemetry/src/global/trace.rs	0.0%	6 Missing ⚠️
opentelemetry/src/trace/noop.rs	0.0%	4 Missing ⚠️
opentelemetry-sdk/src/trace/links.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1515     +/-   ##
=======================================
- Coverage   68.9%   68.8%   -0.1%     
=======================================
  Files        137     137             
  Lines      19799   19844     +45     
=======================================
+ Hits       13658   13671     +13     
- Misses      6141    6173     +32     

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[hdost]

Looks good for the most part. I would have to look at the commit log but if this Link api was released in a previous version of the opentelemetry crate that we should have a CHANGELOG entry for the signature change and addition of the with_context

[TommyCpp]

We need to wrap this API with otel_unstable as it's experimental

[cijothomas]

We need to wrap this API with otel_unstable as it's experimental

Do we need to? The spec is about to get stable soon, so its not worth the effort to hide it under feature-flag now and later undo that.

[TommyCpp]

Do we need to? The spec is about to get stable soon, so its not worth the effort to hide it under feature-flag now and later undo that.

Emm shall we wait for the spec PR to merge before we merge this one then? I am mostly worried we merge this and then spec made some changes before stablizing. Not sure how long it usually takes for spec to stablize

[cijothomas]

Do we need to? The spec is about to get stable soon, so its not worth the effort to hide it under feature-flag now and later undo that.

Emm shall we wait for the spec PR to merge before we merge this one then? I am mostly worried we merge this and then spec made some changes before stablizing. Not sure how long it usually takes for spec to stablize

We are still pre 1.0, so its still okay? Or we can hold off from merging for few days as well and see if the spec actually gets stable.

[lalitb]

We are still pre 1.0, so its still okay? Or we can hold off from merging for few days as well and see if the spec actually gets stable.

Yeah, will add the otel_unstable flag if it is not stable after (say) a couple of weeks.

[lalitb]

The specs is marked as stable now - open-telemetry/opentelemetry-specification#3887. Should be good to review it now.

[TommyCpp]

one nit otherwise LGTM