chore(otel): adds an API for generating span links

[mabdinur]

What does this PR do?

Adds AddLink to the span struct.

Motivation

Currently span links can only be set on spans on span creation (via WithSpanLinks option). To support many use cases we need the ability to add SpanLinks after a span has been created.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-03-18 14:59:18

Comparing candidate commit b18de31 in PR branch munir/add-span-link-public-api with baseline commit 0730ca1 in branch v2-dev.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 35 metrics, 2 unstable metrics.

scenario:BenchmarkPartialFlushing/Enabled-24

• 🟥 execution_time [+7.389ms; +11.859ms] or [+2.494%; +4.003%]

[mabdinur]

@dianashevchenko Do I also need to update mockspan.go?

[dianashevchenko]

Nits

[dianashevchenko]

@mabdinur just remembered this change should probably be reflected in the opentelemetry package as well. Since, OTel API does have the AddLink method, which we couldn't support before.

[mabdinur]

@mabdinur just remembered this change should probably be reflected in the opentelemetry package as well. Since, OTel API does have the AddLink method, which we couldn't support before.

I am not sure if the Otel API exposes an AddLink method: https://github.com/open-telemetry/opentelemetry-go/blob/main/trace/trace.go#L338. I think adding links to existing spans is a concept internal to the go sdk: https://github.com/open-telemetry/opentelemetry-go/blob/da047e70ef583efbc91883dd2e35c616569260ef/sdk/trace/span.go#L632C25-L632C32.

[dianashevchenko]

@mabdinur yes, I think you're right. Even better then.

@darccio the PR looks good to me, but feel free to review.

[dianashevchenko]

@mabdinur please address my previous comments before marking the conversation as resolved 🙌

[darccio]

LGTM