feat(ddtrace/opentelemetry): add AddLink & RecordError

[ablaine]

What does this PR do?

This implements AddLink and RecordError in the opentelemetry to DD adaptor span.

Motivation

I am trying to use OTel's AddLink and discovered that it currently delegates to the noop.Span handler. I temporarily swapped noop.Span with embedded.Span to also discover RecordError was missing.

I looked at OpenTelemetry's RecordError implementation's for the appropriate attributes (exception.type, exception.message and exception.stacktrace), for example: sdk/trace/span.go

For AddLink, I re-used the basic logic from ddtrace/opentelemetry/tracer.go.

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.
• New code is free of linting errors. You can check this by running make lint locally.
• New code doesn't break existing tests. You can check this by running make test locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• All generated files are up to date. You can check this by running make generate locally.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild. Make sure all nested modules are up to date by running make fix-modules locally.

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[ablaine]

side note, it looks as if s.events is missing mutex protection when being written on this line. Let me know and I am happy to add lock/unlock to the AddEvent method.