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: 2025-09-05 19:39:42

Comparing candidate commit 25440fd in PR branch hannahkm/msgp-payload-interface with baseline commit ef95cbd in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.

[datadog-official]

⚠️ Tests

⚠️ Warnings

🧪 1 Test failed

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

Failed

=== RUN   TestTraceProtocol
    writer_test.go:462: 
        	Error Trace:	D:/a/dd-trace-go/dd-trace-go/ddtrace/tracer/writer_test.go:462
        	Error:      	Not equal: 
        	            	expected: 1
        	            	actual  : 0.4
        	Test:       	TestTraceProtocol
--- FAIL: TestTraceProtocol (0.01s)
...

ℹ️ Info

❄️ No new flaky tests detected

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