[Draft; don't merge] v2.0.0-rc

[darccio]

What does this PR do?

This PR is a preview of the changes that will be merged into main for v2 release as github.com/DataDog/dd-trace-go/v2.

Motivation

These are the main motivations behind releasing a v2:

• Migrate from gopkg.in: this causes pain in enterprise customers.
• Long due refactor: moving away from interfaces in favor of concrete types in the API is a requirement to avoid situations where we had to hack a solution because adding functions caused breaking changes.
• Mitigate false-positive results from third party security scanners: due to our bloated go.mod pulling contrib’s dependencies. Also, remove deprecated dependencies’ contribs.
• Reduce library misuse: introduce better patterns, like using context to start spans to ensure trace propagation.
• Long backlog of proposals blocked waiting for a never-getting-planned v2.

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.

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: 2025-01-31 16:29:32

Comparing candidate commit 36f7fad in PR branch v2-dev with baseline commit 4f57a47 in branch main.

Some scenarios are present only in baseline or only in candidate runs. If you didn't create or remove some scenarios in your branch, this maybe a sign of crashed benchmarks 💥💥💥
Check Gitlab CI job log to find if any benchmark has crashed.

Scenarios present only in baseline:

• BenchmarkSampleWAFContext-24
• BenchmarkStartRequestSpan-24

Found 2 performance improvements and 5 performance regressions! Performance is the same for 45 metrics, 1 unstable metrics.

scenario:BenchmarkHttpServeTrace-24

• 🟥 allocations [+3; +3] or [+3.797%; +3.797%]
• 🟥 execution_time [+627.548ns; +705.452ns] or [+3.930%; +4.418%]

scenario:BenchmarkInjectW3C-24

• 🟥 execution_time [+92.182ns; +111.218ns] or [+2.303%; +2.779%]

scenario:BenchmarkSetTagStringer-24

• 🟩 execution_time [-8.450ns; -5.410ns] or [-5.570%; -3.566%]

scenario:BenchmarkStartSpan-24

• 🟥 execution_time [+72.332ns; +96.068ns] or [+3.238%; +4.300%]

scenario:BenchmarkTracerAddSpans-24

• 🟥 allocated_mem [+56 bytes; +57 bytes] or [+2.454%; +2.477%]
• 🟩 execution_time [-144.860ns; -101.340ns] or [-3.506%; -2.453%]

[github-actions]

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