opentelemetry: refactor span links code for clarity

[katiehockman]

What does this PR do?

This change also includes a number of refactors, including some additional information about the trace flags.

Motivation

We should maintain some of the logic in the original PR when creating the DD span link: https://github.com/DataDog/dd-trace-go/pull/2396/files#diff-328a045c801a15647d453336c495d31dcd8016d29df86de478c77769d6792567

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!

[mabdinur]

Left one question. Looks good to me! Thanks for catching these issues

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-02-02 18:12:36

Comparing candidate commit 19cfab9 in PR branch katie.hockman/span-links with baseline commit bbbbd7a in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 40 metrics, 1 unstable metrics.