draft: implement v1 trace protocol serialization

[hannahkm]

• Added msgp struct tags to fields in payloadV1 and traceChunk for serialization.
• Introduced EncodeMsg methods for payloadV1 and spanListV1 to support msgp encoding.
• Updated field types in traceChunk to enhance compatibility with msgp serialization.

What does this PR do?

Implements the v1 trace protocol. Includes commits cherry-picked from #3932 since I closed my previous PR.

Motivation

https://datadoghq.atlassian.net/browse/LANGPLAT-750

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: 2025-09-29 19:12:00

Comparing candidate commit a12502c in PR branch hannahkm/implement-v1-serialization with baseline commit 25440fd in branch hannahkm/msgp-payload-interface.

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

[datadog-official]

⚠️ Tests

⚠️ Warnings

❄️ 1 New flaky test detected

TestWrapConsumerGroupHandler from github.com/DataDog/dd-trace-go/contrib/IBM/sarama/v2 (Datadog) (✨ Fix with BitsAI)

Failed

=== RUN   TestWrapConsumerGroupHandler
2025/09/29 18:59:04 Sarama consumer up and running!...
    consumer_group_test.go:146: Message claimed: value = test 1, timestamp = 2025-09-29 18:59:04.742 +0000 UTC, topic = IBM_sarama_TestWrapConsumerGroupHandler
    consumer_group_test.go:88: 
        	Error Trace:	/home/runner/work/dd-trace-go/dd-trace-go/contrib/IBM/sarama/consumer_group_test.go:88
        	Error:      	"[
        	            	name: kafka.produce
        	            	tags: map[string]interface {}{"_dd.base_service":"", "_dd.p.tid":"68dad6f800000000", "_dd.profiling.enabled":0, "_dd.top_level":1, "component":"IBM/sarama", "language":"go", "messaging.destination.name":"IBM_sarama_TestWrapConsumerGroupHandler", "messaging.kafka.partition":0, "messaging.system":"kafka", "offset":0, "resource.name":"Produce Topic IBM_sarama_TestWrapConsumerGroupHandler", "service.name":"kafka", "span.kind":"producer", "span.name":"kafka.produce", "span.type":"queue"}
...

ℹ️ Info

🧪 All tests passed

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[darccio]

I'm not aware of any concurrent serialization going over the same span. Do we need this?

[darccio]

Beyond the linter errors, this is looking in the right direction.