ddtrace/tracer: Ensure SpanLink access is safe from corruption

[mtoffl01]

What does this PR do?

• Changes the (spanContext).SpanLinks() implementation to return a copy of a slice instead of the slice itself
• Removes (spanContext).SetLinks() method
• nit: Clean up a test

Motivation

Ensure users cannot modify spanContext

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.

Unsure? Have a question? Request a review!

[mhlidd]

Should an assertion be made here to make sure that the casting doesn't fail? Otherwise what is the point of this declaration? 🤔

[mtoffl01]

The code won't compile if this fails; this is a common code pattern for "checking that a concrete type fulfills an interface" in Go.
https://dev.to/kittipat1413/checking-if-a-type-satisfies-an-interface-in-go-432n#:~:text=In%20Go%2C%20to%20check%20if,now%2C%20not%20at%20runtime.%E2%80%9D

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-11-11 16:47:59

Comparing candidate commit bf2289d in PR branch mtoff/safeguard-spanlinks with baseline commit 16269d5 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 59 metrics, 0 unstable metrics.