tracing: Adds support for Span Links

[mabdinur]

What does this PR do?

Implements Span Links in the opentelemetry datadog API

Builds on: https://github.com/DataDog/dd-trace-go/pull/2396/files#diff-a8dc5a7b943aca594d89dc56beced16d9dbc951c6aa0c0ce2d328432dab92c80.

Motivation

Previously setting a span link on an OpenTelemetry span would result in a no-op with this change SpanLinks will be serialized and sent to the Datadog Agent.

Testing

Run the following snippet with a version of the datadog agent that support decoding v0.4 span links:

	tracer.Start(tracer.WithAgentAddr("localhost:8136"))
	defer tracer.Stop()

	// Start a root span
	links := []ddtrace.SpanLink{{TraceID: 1, SpanID: 2}, {TraceID: 3, SpanID: 4, Attributes: map[string]string{"link.name": "alpha_transaction"}, Tracestate: "dd_origin=ci", Flags: 0x80000000}}
	span := tracer.StartSpan("span_with_link", tracer.WithSpanLinks(links))
	span.Finish()
	tracer.Flush()

This generates the following trace level logs in the agent:

2024-02-01 14:24:36 EST | TRACE | TRACE | (pkg/trace/writer/trace.go:209 in addSpans) | Writer: handling new tracer payload with 1 spans: languageName:"go" languageVersion:"1.21.6" tracerVersion:"v1.61.0-dev" chunks:{priority:1 spans:{service:"ddtrace.test" name:"span_with_link" resource:"span_with_link" traceID:6389928532662419093 spanID:6389928532662419093 start:1706815476827049000 duration:34000 meta:{key:"_dd.install.id" value:"35a8c1f1-946f-4a04-b67d-677fb2fd5bd4"} meta:{key:"_dd.install.time" value:"1705966150"} meta:{key:"_dd.install.type" value:"manual"} meta:{key:"_dd.p.dm" value:"-1"} meta:{key:"_dd.p.tid" value:"65bbeff400000000"} meta:{key:"language" value:"go"} meta:{key:"runtime-id" value:"2ee0c4f1-532d-45f5-b7ad-342011775d6c"} metrics:{key:"_dd.agent_psr" value:1} metrics:{key:"_dd.profiling.enabled" value:0} metrics:{key:"_dd.top_level" value:1} metrics:{key:"_dd.trace_span_attribute_schema" value:0} metrics:{key:"_sampling_priority_v1" value:1} metrics:{key:"_top_level" value:1} metrics:{key:"process_id" value:76678} spanLinks:{traceID:1 spanID:2} spanLinks:{traceID:3 spanID:4 attributes:{key:"link.name" value:"alpha_transaction"} tracestate:"dd_origin=ci" flags:2147483648}}}

These links are currently shown in the Datadog UI as "hidden span attributes":

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.

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: 2024-02-01 19:10:30

Comparing candidate commit c75a095 in PR branch munir/implement-span-links-poc with baseline commit e707cc9 in branch main.

Found 0 performance improvements and 12 performance regressions! Performance is the same for 27 metrics, 2 unstable metrics.

scenario:BenchmarkOTelApiWithCustomTags/datadog_otel_api-24

• 🟥 allocated_mem [+76 bytes; +84 bytes] or [+2.101%; +2.326%]

scenario:BenchmarkPartialFlushing/Disabled-24

• 🟥 allocated_mem [+9.143MB; +9.430MB] or [+3.014%; +3.108%]

scenario:BenchmarkPartialFlushing/Enabled-24

• 🟥 allocated_mem [+9.313MB; +13.147MB] or [+2.411%; +3.404%]

scenario:BenchmarkSingleSpanRetention/no-rules-24

• 🟥 allocated_mem [+4.848KB; +4.849KB] or [+3.087%; +3.087%]
• 🟥 execution_time [+9.682µs; +11.064µs] or [+3.901%; +4.457%]

scenario:BenchmarkSingleSpanRetention/with-rules/match-all-24

• 🟥 allocated_mem [+4.732KB; +4.906KB] or [+2.968%; +3.077%]
• 🟥 execution_time [+9.372µs; +10.696µs] or [+3.745%; +4.274%]

scenario:BenchmarkSingleSpanRetention/with-rules/match-half-24

• 🟥 allocated_mem [+4.700KB; +4.895KB] or [+2.948%; +3.071%]
• 🟥 execution_time [+8.437µs; +10.194µs] or [+3.364%; +4.064%]

scenario:BenchmarkStartSpan-24

• 🟥 allocated_mem [+48 bytes; +48 bytes] or [+3.076%; +3.128%]

scenario:BenchmarkTracerAddSpans-24

• 🟥 allocated_mem [+49 bytes; +49 bytes] or [+2.212%; +2.212%]
• 🟥 execution_time [+85.364ns; +131.036ns] or [+2.110%; +3.239%]

[github-actions]

Remaining comments which cannot be posted as a review comment to avoid GitHub Rate Limit

golangci

ddtrace/ddtrace_gen.go|714 col 23| z.Parent.Msgsize undefined (type SpanContext has no field or method Msgsize)
ddtrace/ddtrace_gen.go|721 col 43| z.Context.Msgsize undefined (type context.Context has no field or method Msgsize)
ddtrace/ddtrace_gen.go|721 col 43| too many errors) (typecheck)
ddtrace/ddtrace_gen.go|393 col 19| z.Parent.DecodeMsg undefined (type SpanContext has no field or method DecodeMsg)
ddtrace/ddtrace_gen.go|441 col 20| z.Context.DecodeMsg undefined (type context.Context has no field or method DecodeMsg)
ddtrace/ddtrace_gen.go|484 col 17| z.Parent.EncodeMsg undefined (type SpanContext has no field or method EncodeMsg)
ddtrace/ddtrace_gen.go|536 col 18| z.Context.EncodeMsg undefined (type context.Context has no field or method EncodeMsg)
ddtrace/ddtrace_gen.go|567 col 20| z.Parent.MarshalMsg undefined (type SpanContext has no field or method MarshalMsg)
ddtrace/ddtrace_gen.go|591 col 21| z.Context.MarshalMsg undefined (type context.Context has no field or method MarshalMsg)
ddtrace/ddtrace_gen.go|628 col 24| z.Parent.UnmarshalMsg undefined (type SpanContext has no field or method UnmarshalMsg)
ddtrace/ddtrace_gen.go|676 col 25| z.Context.UnmarshalMsg undefined (type context.Context has no field or method UnmarshalMsg)
ddtrace/ddtrace_gen.go|714 col 23| z.Parent.Msgsize undefined (type SpanContext has no field or method Msgsize)
ddtrace/ddtrace_gen.go|721 col 43| z.Context.Msgsize undefined (type context.Context has no field or method Msgsize)
ddtrace/ddtrace_gen.go|721 col 43| too many errors (typecheck)
ddtrace/ddtrace_gen.go|393 col 19| z.Parent.DecodeMsg undefined (type SpanContext has no field or method DecodeMsg)
ddtrace/ddtrace_gen.go|441 col 20| z.Context.DecodeMsg undefined (type context.Context has no field or method DecodeMsg)
ddtrace/ddtrace_gen.go|484 col 17| z.Parent.EncodeMsg undefined (type SpanContext has no field or method EncodeMsg)
ddtrace/ddtrace_gen.go|536 col 18| z.Context.EncodeMsg undefined (type context.Context has no field or method EncodeMsg)
ddtrace/ddtrace_gen.go|567 col 20| z.Parent.MarshalMsg undefined (type SpanContext has no field or method MarshalMsg)
ddtrace/ddtrace_gen.go|591 col 21| z.Context.MarshalMsg undefined (type context.Context has no field or method MarshalMsg)
ddtrace/ddtrace_gen.go|628 col 24| z.Parent.UnmarshalMsg undefined (type SpanContext has no field or method UnmarshalMsg)
ddtrace/ddtrace_gen.go|676 col 25| z.Context.UnmarshalMsg undefined (type context.Context has no field or method UnmarshalMsg)
ddtrace/ddtrace_gen.go|714 col 23| z.Parent.Msgsize undefined (type SpanContext has no field or method Msgsize)
ddtrace/ddtrace_gen.go|721 col 43| z.Context.Msgsize undefined (type context.Context has no field or method Msgsize)
ddtrace/ddtrace_gen.go|721 col 43| too many errors) (typecheck)

[github-actions]

Remaining comments which cannot be posted as a review comment to avoid GitHub Rate Limit

golangci

ddtrace/span_link_msgp.go|363 col 19| z.Parent.DecodeMsg undefined (type SpanContext has no field or method DecodeMsg)
ddtrace/span_link_msgp.go|411 col 20| z.Context.DecodeMsg undefined (type context.Context has no field or method DecodeMsg)
ddtrace/span_link_msgp.go|454 col 17| z.Parent.EncodeMsg undefined (type SpanContext has no field or method EncodeMsg)
ddtrace/span_link_msgp.go|506 col 18| z.Context.EncodeMsg undefined (type context.Context has no field or method EncodeMsg)
ddtrace/span_link_msgp.go|533 col 23| z.Parent.Msgsize undefined (type SpanContext has no field or method Msgsize)
ddtrace/span_link_msgp.go|540 col 43| z.Context.Msgsize undefined (type context.Context has no field or method Msgsize)) (typecheck)

[Julio-Guerra]

I removed asm-go from the reviewers as I didn't see anything other than pure APM tracing in this PR.
Let me know if not and need our review.

[katiehockman]

A few nits for some Go idioms, but otherwise looks good. Thanks!