feat: initialize v1 trace protocol with interface

[hannahkm]

What does this PR do?

Motivation

https://datadoghq.atlassian.net/browse/LANGPLAT-709

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 ./scripts/lint.sh locally.
• 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!

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-10-16 09:30:11

Comparing candidate commit bbb62d2 in PR branch hannahkm/msgp-payload-interface with baseline commit 9884fb8 in branch main.

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

scenario:BenchmarkInjectW3C-24

• 🟥 execution_time [+88.377ns; +115.423ns] or [+2.250%; +2.938%]

[datadog-official]

⚠️ Tests

⚠️ Warnings

🧪 2 Tests failed

TestTraceProtocol from github.com/DataDog/dd-trace-go/v2/ddtrace/tracer (Datadog)

Failed

=== RUN   TestTraceProtocol
--- FAIL: TestTraceProtocol (0.00s)

TestTraceProtocol/v1.0 from github.com/DataDog/dd-trace-go/v2/ddtrace/tracer (Datadog)

Failed

=== RUN   TestTraceProtocol/v1.0
    --- FAIL: TestTraceProtocol/v1.0 (0.00s)
panic: not implemented [recovered, repanicked]

goroutine 20411 [running]:
testing.tRunner.func1.2({0x7ff7e29bd440, 0x7ff7e2d966b0})
	C:/hostedtoolcache/windows/go/1.25.3/x64/src/testing/testing.go:1872 +0x239
testing.tRunner.func1()
...

ℹ️ Info

❄️ No new flaky tests detected

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